35 CAUSATIVE PATHOGEN

# 36 CAUSATIVE PATHOGEN

Why this topic matters

"Match the bug to the scenario" is one of the highest-yield SBA patterns in MRCS Part A. The examiner gives you a one-line vignette — a sickle cell child with osteomyelitis, a watercress farmer with jaundice, an IVDU with cavitating lung lesions — and expects you to pattern-match instantly. There is rarely a need to reason from microbiology first principles; what is needed is a library of trigger words mapped to organisms.

This lesson is built around that library. The master table below collects the highest-yield scenario→organism pairs. The detailed notes that follow add the why (mechanism, host factors, complications) so that recall is robust rather than rote.

Master table — scenario to organism

Clinical scenario	Most likely pathogen	Trigger word
Surgical site infection (early, < 48 h)	Strep pyogenes	Spreading erythema
Surgical site infection (typical, 4–7 d)	S. aureus (incl. MRSA)	Purulent wound
Cellulitis	Strep pyogenes > S. aureus	Lymphoedema, insect bite
Necrotising fasciitis type I	Polymicrobial (anaerobes + GNRs)	Diabetic, perineum (Fournier's)
Necrotising fasciitis type II	Strep pyogenes (Group A)	Healthy adult, rapid limb
Gas gangrene	Clostridium perfringens	Crepitus, dirty wound, diabetic stump
Tetanus	Clostridium tetani	Rusty nail, trismus, opisthotonos
Cholecystitis / cholangitis	E. coli > Klebsiella, Enterococcus	RUQ, jaundice, Charcot's triad
Community-acquired UTI	E. coli (~75%)	Dysuria, frequency
Catheter-associated UTI	Pseudomonas aeruginosa, Proteus, Candida	Long-term catheter
Staghorn calculus	Proteus mirabilis	Urease, alkaline urine, struvite
Community-acquired pneumonia	Strep pneumoniae	Lobar consolidation, rust sputum
Hospital-acquired pneumonia	Pseudomonas, Klebsiella	ITU, ventilated
Aspiration pneumonia	Anaerobes ± Gram-negatives	Stroke, alcoholic, RLL
Post-transplant pneumonia (1–6 mo)	CMV	Solid organ transplant
Cavitating apical lung lesion	TB; S. aureus post-flu; Klebsiella in alcoholics	Weight loss, night sweats
Osteomyelitis (general)	S. aureus	—
Osteomyelitis in sickle cell	Salmonella	HbSS child, long bone
Discitis (adult)	S. aureus	—
Discitis (6 mo – 4 yr)	Kingella kingae	Toddler, limp
Septic arthritis (adult)	S. aureus	Hot swollen joint
Septic arthritis (young sexually active)	Neisseria gonorrhoeae	Migratory arthralgia, tenosynovitis
Native valve endocarditis	Strep viridans	Dental procedure, subacute
IVDU endocarditis (tricuspid)	S. aureus	Septic pulmonary emboli
Prosthetic valve, early (< 2 mo)	S. epidermidis	Coagulase-negative
Prosthetic valve, late (> 1 yr)	S. aureus, Strep	—
Mycotic aneurysm	S. aureus	—
C. difficile colitis	C. difficile	Post-antibiotic diarrhoea, PMC
MRSA	MRSA	Vancomycin
Typhoid (enteric fever)	Salmonella typhi	Step-ladder fever, rose spots, splenomegaly, gastric perforation week 3
Amoebic dysentery / liver abscess	Entamoeba histolytica	Travel, bloody diarrhoea, anchovy-paste pus
Hydatid disease	Echinococcus granulosus	Sheep farmer, liver cyst, anaphylaxis if ruptured
Schistosomiasis (urinary)	Schistosoma haematobium	Egyptian/African, haematuria, bladder SCC
Liver fluke + watercress	Fasciola hepatica	Eosinophilia, biliary
Cat bite	Pasteurella multocida	Rapid cellulitis < 24 h
Human bite	Eikenella corrodens (+ Strep, anaerobes)	Fight bite, knuckle
Dog bite	Pasteurella, Capnocytophaga	Asplenic → fulminant
Cutaneous anthrax	Bacillus anthracis	Farmer, painless black eschar
Actinomycosis	Actinomyces israelii	Sulphur granules, post-dental, sinuses
Acute otitis media	S. pneumoniae > H. influenzae > Moraxella	Post-URTI child
Epiglottitis	Haemophilus influenzae type b	Drooling, tripod, thumbprint
Epididymo-orchitis (< 35 y)	Chlamydia trachomatis	Sexually active, Prehn's positive
Epididymo-orchitis (> 35 y)	E. coli	UTI organisms
Mycobacterium avium (HIV)	MAI	CD4 < 50
Needlestick — highest risk	Hepatitis B (~30%)	HBeAg positive source
Splenic rupture, no trauma	EBV	Glandular fever, contact sport
Nasopharyngeal carcinoma	EBV	Southern Chinese
Kaposi's sarcoma	HHV-8	AIDS
Oropharyngeal / cervical cancer	HPV (16, 18)	—

The rest of this lesson reinforces the why behind these matches.

Skin and soft tissue infections

Cellulitis is a diffuse infection of dermis and subcutaneous fat. Strep pyogenes (Group A Strep) is the dominant pathogen, particularly when there is broken skin, lymphoedema, or an insect bite. S. aureus is the second commonest and is more typical when there is a focal collection (e.g. abscess, folliculitis).

Surgical site infection (SSI) is dominated by skin flora. S. aureus (including MRSA) is the single commonest organism overall. A very early SSI (within 48 hours) with spreading erythema and bullae should raise suspicion of Group A Strep, which is one of the few organisms that can produce a fulminant wound infection on day one.

👩‍⚕️ A spreading red wound at 24–48 h post-op is Strep until proved otherwise. Frank pus at 4–7 days is Staph.

Necrotising fasciitis

A surgical emergency. Two types you must distinguish:

➡ Type I — polymicrobial. Mixed aerobes and anaerobes (E. coli, Bacteroides, Clostridium, Enterococcus). Diabetics, immunosuppressed, post-operative abdominal wounds. Fournier's gangrene (necrotising fasciitis of the perineum) is the classic type I presentation.

➡ Type II — monomicrobial. Group A Strep (Strep pyogenes), occasionally with S. aureus. Healthy young adults, often a limb, rapidly progressive over hours.

Empirical antibiotics: broad-spectrum (e.g. meropenem) plus clindamycin. Clindamycin is added specifically to switch off streptococcal exotoxin production at the ribosome — antibiotic choice is a favourite SBA point.

Gas gangrene

Clostridium perfringens — an anaerobic, spore-forming Gram-positive rod. Trigger words: crepitus, dirty traumatic wound, diabetic amputation stump, air in soft tissues on X-ray. Toxin (alpha-toxin, a lecithinase) destroys cell membranes and produces gas as it ferments tissue. Treatment is aggressive surgical debridement plus benzylpenicillin and clindamycin.

Tetanus

Clostridium tetani. Spores enter via a contaminated puncture wound (classic: rusty nail, gardening injury). Toxin (tetanospasmin) travels retrograde up motor neurones and blocks inhibitory neurotransmitters → unopposed muscle contraction → trismus (lockjaw), risus sardonicus, opisthotonos.

Bites

Bite	Classic organism	Note
Cat	Pasteurella multocida	Cellulitis within hours; deep puncture wounds seed tendon sheaths
Dog	Pasteurella, Capnocytophaga canimorsus	Capnocytophaga is fulminant in asplenic patients
Human ("fight bite")	Eikenella corrodens + Strep, anaerobes	Knuckle wound from punching teeth; high complication rate

Co-amoxiclav covers all three. Tetanus and rabies status must be addressed.

Cutaneous anthrax

Bacillus anthracis — Gram-positive spore-forming rod. Farmers, wool sorters, abattoir workers. The lesion is the giveaway: painless papule → vesicle → painless black eschar surrounded by a rose-pink rim of induration. Painlessness despite an angry-looking lesion is the trap — bacterial skin infections normally hurt.

Intra-abdominal and biliary infections

The biliary tree, large bowel and urinary tract share an organism profile dominated by Gram-negative enterics:

➡ E. coli — top of every intra-abdominal list (cholecystitis, cholangitis, diverticulitis, appendicitis, peritonitis, UTI).

➡ Klebsiella, Enterobacter, Proteus — also Gram-negative enterics; Proteus is notable for urease and staghorn calculi.

➡ Enterococcus — Gram-positive, enters the picture in biliary sepsis and in patients on cephalosporins (which don't cover it).

➡ Anaerobes (Bacteroides fragilis) — co-pathogens in colonic and pelvic sepsis.

C. difficile

Post-antibiotic diarrhoea, classically 5–10 days after the offending course. The four C's that trigger it: clindamycin, ciprofloxacin, co-amoxiclav, cephalosporins. Toxins A and B cause pseudomembranous colitis. Spread is faeco-oral via spores — alcohol gel does NOT kill spores, soap and water do. Treatment: oral vancomycin (first line) or fidaxomicin; metronidazole is now second line.

Urinary tract

E. coli causes ~75% of community-acquired UTIs (uropathogenic E. coli with P-fimbriae adhering to uroepithelium). In catheterised or hospitalised patients the spectrum shifts to Pseudomonas, Proteus, Klebsiella, Enterococcus and Candida. Pseudomonas thrives in biofilm on plastic and is multi-drug resistant.

Proteus mirabilis is the urease producer — it splits urea to ammonia, alkalinises urine, and precipitates struvite (magnesium ammonium phosphate) stones that grow into staghorn calculi.

Schistosoma haematobium

A trematode parasite endemic in rural Egypt and sub-Saharan Africa. Eggs lodge in the bladder wall, causing terminal haematuria, irritative symptoms, calcification, and — over years — squamous metaplasia → squamous cell carcinoma of the bladder. This is the only major SCC of the bladder you need to know; UK bladder cancer is overwhelmingly transitional cell.

Respiratory infections

Setting	Pathogens
Community-acquired (CAP)	Strep pneumoniae (commonest), H. influenzae, Mycoplasma, Legionella
Hospital-acquired (HAP, > 48 h)	Pseudomonas, Klebsiella, MRSA
Aspiration	Mouth flora — anaerobes + Gram-negatives
Post-influenza	S. aureus (cavitates)
Cystic fibrosis	Pseudomonas, Staph, Burkholderia
Sickle cell pneumonia	Strep pneumoniae (encapsulated, asplenic patient)

Cavitating apical lung lesions

➡ TB — caseating necrosis, Langhans giant cells, Ghon focus.

➡ S. aureus — post-influenza necrotising pneumonia → empyema.

➡ Klebsiella — alcoholics, "currant-jelly" sputum.

➡ Aspergilloma — fungal ball in pre-existing cavity, air-crescent sign.

➡ Septic pulmonary emboli — multiple peripheral cavities, tricuspid endocarditis in IVDUs.

➡ Squamous cell carcinoma — central necrosis in a smoker's upper-lobe mass.

➡ GPA (Wegener's) — c-ANCA positive, necrotising granulomas.

Epiglottitis

Haemophilus influenzae type b. Drooling, tripod posture, muffled voice, thumbprint sign on lateral neck X-ray. Now rare in vaccinated populations.

Acute otitis media (children)

Top three, in order: Strep pneumoniae > H. influenzae > Moraxella catarrhalis. Often follows a viral URTI.

Bone and joint

S. aureus is the default for osteomyelitis and septic arthritis at every age — except where a special host changes the rules:

➡ Sickle cell disease → Salmonella osteomyelitis (functional asplenia + microinfarcts seed long bones).

➡ Children 6 months – 4 years → Kingella kingae discitis and septic arthritis.

➡ Sexually active young adult with migratory arthralgia, tenosynovitis, pustular rash → Neisseria gonorrhoeae.

➡ Vertebral osteomyelitis with a smouldering course → consider TB (Pott's disease) or Brucella (unpasteurised dairy).

👩‍⚕️ Sickle cell patients are functionally asplenic and susceptible to encapsulated organisms: Strep pneumoniae (commonest infection overall), Hib, Neisseria meningitidis. Salmonella is the exam answer for osteomyelitis specifically.

Infective endocarditis

Setting	Organism
Native valve, subacute (dental)	Strep viridans
Native valve, acute	S. aureus
IVDU (tricuspid)	S. aureus
Prosthetic valve, early (< 2 mo)	Staph epidermidis (coag-negative, biofilm)
Prosthetic valve, late (> 1 yr)	S. aureus, Strep
Colon cancer association	Strep gallolyticus (formerly S. bovis) — always colonoscope
Culture-negative	HACEK, Coxiella, Bartonella

Mycotic aneurysm — embolic infectious aneurysm — is most commonly due to S. aureus, particularly in IVDU endocarditis.

Tropical and travel-related infections

Typhoid (enteric fever)

Salmonella typhi. Travel to South Asia. Week 1: step-ladder fever, relative bradycardia (Faget's sign). Week 2: rose spots, splenomegaly, abdominal pain. Week 3: complications — terminal ileal Peyer's patch necrosis → perforation and haemorrhage. This is the typhoid scenario the MRCS examiner wants you to recognise: a returning traveller with high fever, splenomegaly and a surgical abdomen.

Amoebic liver abscess

Entamoeba histolytica. Travel, bloody diarrhoea (flask-shaped colonic ulcers), then RUQ pain and a single liver abscess in the right lobe with "anchovy-paste" contents. Treat with metronidazole then a luminal agent (paromomycin).

Hydatid disease

Echinococcus granulosus. Sheep-farming areas (Mediterranean, Middle East, Wales). Cysts in liver > lung. Surgery requires albendazole pre-op and meticulous technique — spillage causes anaphylaxis and seeding.

Fasciola hepatica

Watercress farmers, biliary obstruction, eosinophilia. Treatment: triclabendazole.

Malaria

Plasmodium falciparum is the dangerous one. Always on the differential for fever in a returning traveller. Surgically relevant complication: splenic rupture.

Transplant and immunocompromised hosts

Timeline matters:

➡ < 1 month post-transplant → nosocomial pathogens, wound infection, line sepsis.

➡ 1–6 months → opportunistic: CMV (commonest cause of post-transplant pneumonia), PCP, BK virus.

➡ > 6 months / years → EBV-driven post-transplant lymphoproliferative disorder (PTLD), late CMV, community pathogens.

EBV — the great mimicker

A single virus with a long list of associations:

➡ Infectious mononucleosis (heterophile/Monospot positive, atypical lymphocytes).

➡ Spontaneous splenic rupture (avoid contact sports for 4–6 weeks).

➡ Burkitt's lymphoma.

➡ Hodgkin's lymphoma.

➡ Nasopharyngeal carcinoma (Southern Chinese populations).

➡ PTLD.

Other oncogenic viruses

Virus	Cancer
HPV 16, 18	Cervical, anal, oropharyngeal SCC
HHV-8	Kaposi's sarcoma
HBV, HCV	Hepatocellular carcinoma
HTLV-1	Adult T-cell leukaemia/lymphoma
H. pylori	Gastric adenocarcinoma, MALT lymphoma
Schistosoma haematobium	Bladder SCC
EBV	Burkitt's, Hodgkin's, nasopharyngeal, PTLD

HIV opportunistic infections

➡ CD4 < 200 → PCP, oral candida.

➡ CD4 < 100 → Toxoplasma, cryptococcus.

➡ CD4 < 50 → Mycobacterium avium-intracellulare (MAI) — disseminated lymphadenopathy, fever, weight loss.

Needlestick injuries — risk of transmission

Memorise the order: HBV >> HCV >> HIV.

Pathogen	Risk per needlestick
Hepatitis B	Up to 30% (HBeAg-positive source)
Hepatitis C	~3%
HIV	~0.3%

Sterilisation — match the method to the instrument

Instrument	Method
Standard surgical instruments, drapes, culture media	Autoclave (steam, 121 °C, 15 min, 15 psi)
Glassware, oils, powders, metal instruments	Hot air oven (160 °C, 2 h)
Flexible endoscopes (heat-sensitive)	Glutaraldehyde (high-level chemical) or ethylene oxide gas
Disposable plastic syringes	Gamma irradiation
Heat-sensitive liquids (serum, vaccines)	Membrane filtration

Clostridial spores survive boiling at 100 °C — you need full autoclave conditions or sporicidal chemicals to kill them. Glutaraldehyde is the answer for endoscopes; ethylene oxide is the second-best choice when glutaraldehyde is not listed.

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Test yourself

A 54-year-old woman is admitted one week after cholecystectomy with profuse watery diarrhoea. The operation was uncomplicated apart from minor bile spillage. Most likely diagnosis?

• ((Giardia lamblia::Steatorrhoea after travel/contaminated water, not post-op antibiotic exposure.))
• ((E. coli::Causes the biliary infection, but not post-op diarrhoea.))
• ((Clostridium difficile::☑️ Profuse diarrhoea 5–10 days after surgical antibiotics is classic for C. diff.))
• ((Salmonella::Food-borne diarrhoea, not hospital-acquired post-op pattern.))
• ((Pelvic abscess::Presents with fever and pelvic pain, not watery diarrhoea.))

What is the mode of transmission of Clostridium difficile?

• ((Airborne droplet::C. diff is not respiratory.))
• ((Faeco-oral via spores::☑️ Spores resist alcohol gel — wash hands with soap and water.))
• ((Blood-borne::No bloodstream transmission.))
• ((Sexual::Not relevant.))

A diabetic patient who has had a below-knee amputation develops fever, a swollen red stump and crepitus. Most likely diagnosis?

• ((Cellulitis::Would not produce crepitus.))
• ((Gas gangrene (Clostridium perfringens)::☑️ Crepitus + dirty wound + ischaemic tissue = gas gangrene.))
• ((Necrotising fasciitis type II::Group A Strep, no gas production.))
• ((Deep vein thrombosis::No fever or crepitus.))

A man sustains a dirty laceration to the thigh. Several days later the wound is infected with crepitus. Causative organism?

• ((Staphylococcus aureus::Pus and erythema, but no gas.))
• ((Streptococcus pyogenes::Spreading cellulitis, not crepitus.))
• ((Clostridium perfringens::☑️ Anaerobic spore-former; alpha-toxin produces gas in tissue.))
• ((E. coli::Gram-negative, intra-abdominal sepsis.))

What is the best empirical antibiotic combination for necrotising fasciitis?

• ((Co-amoxiclav alone::Inadequate cover for Group A Strep toxin production.))
• ((Meropenem and clindamycin::☑️ Broad cover plus clindamycin to switch off streptococcal exotoxin.))
• ((Ciprofloxacin::No anaerobic cover.))
• ((Vancomycin alone::Misses Gram-negatives and anaerobes.))

👩‍⚕️ Clindamycin's role is anti-toxin, not just antibacterial — a favourite SBA point.

A 53-year-old farmer presents with a painless black eschar on the hand surrounded by a rose-pink indurated rim. Most likely cause?

• ((Staphylococcal abscess::Painful and fluctuant.))
• ((Anthrax (Bacillus anthracis)::☑️ Painless black eschar in someone with animal contact is pathognomonic.))
• ((Necrotising fasciitis::Exquisitely painful, rapidly progressive.))
• ((Orf virus::Vesicular nodule, not eschar.))

A patient with sickle cell disease develops pneumonia. Most likely pathogen?

• ((Streptococcus pneumoniae::☑️ Encapsulated organism in functionally asplenic host.))
• ((Salmonella::Causes osteomyelitis, not pneumonia, in sickle cell.))
• ((Pseudomonas::More typical in CF or HAP.))
• ((Mycoplasma::Possible but not the classic sickle cell answer.))

A woman with post-mastectomy lymphoedema develops a swollen, red, tender arm after an insect bite. Pathogen?

• ((Streptococcus pyogenes::☑️ Classic cause of cellulitis in a lymphoedematous limb.))
• ((Staphylococcus aureus::More often causes focal abscess than diffuse cellulitis.))
• ((Pseudomonas::Hot tub folliculitis or burn wounds.))
• ((Pasteurella multocida::Cat bites.))

Most common pathogen in osteomyelitis overall?

• ((Staphylococcus aureus::☑️ Default answer for bone and joint infection at every age.))
• ((Streptococcus pyogenes::Skin and soft tissue.))
• ((Salmonella::Only in sickle cell.))
• ((E. coli::Vertebral osteomyelitis in elderly UTI patients.))

A 12-year-old boy with sickle cell disease has osteomyelitis of the tibia. Pathogen?

• ((Escherichia coli::Gut/urinary, not bone in sickle cell.))
• ((Neisseria gonorrhoeae::Septic arthritis in sexually active adults.))
• ((Salmonella::☑️ Functional asplenia + bony microinfarcts → Salmonella osteomyelitis.))
• ((Haemophilus influenzae::Epiglottitis and otitis, not bone.))

Most likely pathogen of discitis in a 3-year-old?

• ((Staphylococcus aureus::Commonest overall, but not at this age.))
• ((Kingella kingae::☑️ Age 6 months – 4 years; toddler with limp or refusal to bear weight.))
• ((Salmonella::Sickle cell.))
• ((Streptococcus pneumoniae::Pneumonia and otitis.))

A 35-year-old tennis player suffers an open tibial fracture treated by ORIF. Ten days later the wound is erythematous with discharging pus. Pathogen?

• ((Staphylococcus epidermidis::Indolent infection of indwelling devices, typically > 2 weeks, low-grade.))
• ((Staphylococcus aureus::☑️ Skin flora; commonest pathogen in early post-op orthopaedic infection.))
• ((Pseudomonas aeruginosa::Burns, wet wounds, or immunocompromised.))
• ((Clostridium perfringens::Would produce crepitus and gas.))

An IVDU has a swollen knee. Aspirate grows Gram-positive cocci. Despite lavage and cephalosporins he collapses; CXR shows apical cavitating lesions. Pathogen?

• ((Streptococcus viridans::Subacute native valve endocarditis after dental procedures.))
• ((Staphylococcus aureus::☑️ Tricuspid IVDU endocarditis → septic pulmonary emboli with cavitation.))
• ((Pseudomonas::Rare cause of right-sided endocarditis.))
• ((Candida::Prosthetic valves or TPN.))

A 45-year-old homeless man has a 3-month cough, weight loss and a cavitating right upper lobe lesion. Biopsy shows featureless necrosis with epithelioid macrophages and giant cells. Diagnosis?

• ((Actinomycosis::Sulphur granules and sinus tracts.))
• ((Squamous cell carcinoma::Malignant squamous cells, not granulomas.))
• ((Sarcoidosis::Non-caseating granulomas.))
• ((Tuberculosis::☑️ Caseating (featureless) necrosis with Langhans giant cells is pathognomonic.))

A patient presents 6 months after a tooth extraction with neck pus, multiple sinuses and yellow granules. Diagnosis?

• ((Tuberculosis::Caseating granulomas, no sulphur granules.))
• ((Actinomycosis::☑️ Yellow sulphur granules + chronic dental source = Actinomyces israelii.))
• ((Syphilis::Painless gummata, not sinuses.))
• ((Osteomyelitis::Bone pain, not sulphur granules.))

A sheep farmer presents with a liver cyst. Pathogen?

• ((Entamoeba histolytica::Amoebic liver abscess, not a true cyst.))
• ((Echinococcus granulosus::☑️ Tapeworm carried by dogs and sheep; hydatid liver cysts.))
• ((Fasciola hepatica::Biliary fluke from watercress.))
• ((Schistosoma::Portal hypertension or urinary disease.))

👩‍⚕️ Rupture of a hydatid cyst causes type I hypersensitivity and anaphylaxis — never aspirate blindly.

A watercress farmer presents with jaundice and eosinophilia. Pathogen?

• ((Fasciola hepatica::☑️ Liver fluke acquired via aquatic plants; biliary obstruction with eosinophilia.))
• ((Clonorchis sinensis::Undercooked freshwater fish in East Asia.))
• ((Schistosoma mansoni::Portal hypertension, not biliary obstruction.))
• ((Echinococcus::Cystic, not fluke disease.))

A man recently returned from Bangladesh presents with bloody diarrhoea, crampy abdominal pain, and stool with cysts and trophozoites. Pathogen?

• ((Salmonella typhi::Step-ladder fever and rose spots; rarely bloody diarrhoea.))
• ((Entamoeba histolytica::☑️ Endemic, flask-shaped colonic ulcers, risk of amoebic liver abscess.))
• ((Campylobacter jejuni::Possible but cysts in stool make amoeba the answer.))
• ((E. coli::EHEC can cause bloody diarrhoea but no cysts.))
• ((Staph aureus::Rapid-onset vomiting from toxin.))

A mother of three presents with pruritus ani; sticky tape test shows eggs. Pathogen?

• ((Enterobius vermicularis::☑️ Pinworm; nocturnal perianal itch; tape test positive for eggs.))
• ((Entamoeba histolytica::Bloody diarrhoea, not itch.))
• ((Ascaris lumbricoides::GI or pulmonary symptoms, not perianal itch.))
• ((Strongyloides::Larva currens rash, not perianal eggs.))

Which organism is associated with staghorn calculi?

• ((E. coli::Commonest cause of UTI but not stone-forming.))
• ((Proteus mirabilis::☑️ Urease alkalinises urine → struvite (Mg-NH₄-phosphate) staghorn stones.))
• ((Pseudomonas::Catheter-associated UTI, not stone-forming.))
• ((Klebsiella::Some urease activity but Proteus is the answer.))

A 49-year-old Egyptian man presents with haematuria and 7 months of bladder irritation. Pathogen?

• ((Schistosoma haematobium::☑️ Endemic in Nile Delta; eggs in bladder wall cause haematuria.))
• ((E. coli::Acute cystitis, not 7-month history.))
• ((Mycobacterium tuberculosis::Sterile pyuria, not the classic exam vignette.))
• ((Adenovirus::Haemorrhagic cystitis in immunocompromised.))

A man from Zimbabwe with chronic haematuria has bladder biopsy showing chronic irritation. What cellular change is expected?

• ((Transitional cell carcinoma::Commonest UK bladder cancer but not the schistosoma association.))
• ((Columnar metaplasia::Seen at the trigone normally, not the schistosoma change.))
• ((Dysplasia::Premalignant but not the specific change.))
• ((Squamous metaplasia::☑️ Chronic schistosomal irritation drives squamous metaplasia → bladder SCC.))

A young girl recently recovered from a viral URTI now pulls at her ear with yellow ear discharge. Most likely pathogen?

• ((Streptococcus intermedius::Anaerobic Strep — brain or liver abscesses.))
• ((Staphylococcus aureus::Otitis externa or secondary infection.))
• ((Moraxella catarrhalis::☑️ Common AOM organism after URTI, alongside Strep pneumoniae and Hib.))
• ((Pseudomonas::Malignant otitis externa in diabetics.))

A 30-year-old HIV-positive man has generalised lymphadenopathy and CD4 < 50. Pathogen?

• ((Follicular hyperplasia::Reactive change, not an organism.))
• ((Mycobacterium avium intracellulare (MAI)::☑️ Disseminated infection at CD4 < 50.))
• ((Onchocerciasis::Skin nodules and river blindness.))
• ((Sinus histiocytosis::Reactive node change.))

A microbiologist reports rising cross-infection in catheterised urology patients. Most likely organism?

• ((Clostridium difficile::Gut, not urinary.))
• ((Streptococcus pneumoniae::Respiratory.))
• ((Staphylococcus aureus::Skin and bone.))
• ((Pseudomonas aeruginosa::☑️ Biofilm-forming, antibiotic-resistant; classic catheter UTI organism.))
• ((Escherichia coli::Commonest UTI overall but Pseudomonas dominates catheter-associated cross-infection.))

A 6-year-old has recurrent pneumonias growing Pseudomonas; he had meconium ileus as a neonate. Confirmatory test?

• ((Sweat alanine transaminase::Not a diagnostic test.))
• ((Sweat chloride > 60 mmol/L::☑️ Diagnostic of cystic fibrosis.))
• ((Sweat creatinine::Not a CF test.))
• ((Sweat potassium::Not diagnostic.))
• ((Sweat urea::Not diagnostic.))

A 26-year-old sexually active man has painful scrotal swelling relieved by elevation and fever 39 °C. Pathogen?

• ((Staphylococcus aureus::Skin and bone infections.))
• ((E. coli::Cause in men > 35 with concurrent UTI.))
• ((Chlamydia trachomatis::☑️ Commonest cause of epididymo-orchitis in men under 35.))
• ((Treponema pallidum::Painless syphilitic chancre.))
• ((Clostridium perfringens::Gas gangrene, not orchitis.))

A patient develops pneumonia 4 months after a renal transplant, not responding to antibiotics. Pathogen?

• ((Epstein-Barr virus::Late post-transplant PTLD.))
• ((Tuberculosis::Possible but CMV is the classic answer.))
• ((Cytomegalovirus (CMV)::☑️ Commonest cause of pneumonia 1–6 months post solid organ transplant.))
• ((Haemophilus influenzae::Community-acquired pneumonia.))
• ((Herpes zoster::Skin dermatomes, not pneumonia.))

A patient 18 months after renal transplant presents with lymphadenopathy and a lymphoproliferative process. Most likely organism?

• ((Epstein-Barr virus (EBV)::☑️ Late (> 6 months) post-transplant lymphoproliferative disorder (PTLD).))
• ((Cytomegalovirus::Pneumonia in the 1–6 month window.))
• ((HTLV-1::Adult T-cell leukaemia, but not the transplant association.))
• ((HHV-8::Kaposi's sarcoma in AIDS.))

Commonest cause of epiglottitis?

• ((Haemophilus influenzae type b::☑️ Hib; drooling child in tripod position, thumbprint sign.))
• ((Streptococcus pneumoniae::Otitis and pneumonia.))
• ((Staphylococcus aureus::Not epiglottitis.))
• ((Moraxella::Otitis media.))

A patient suffers atraumatic splenic rupture with massive splenomegaly. Pathogen?

• ((Measles::No splenomegaly.))
• ((Mumps::Parotitis and orchitis.))
• ((Epstein-Barr virus::☑️ Infectious mononucleosis enlarges and softens the spleen — avoid contact sport 4–6 weeks.))
• ((Cytomegalovirus::Possible mononucleosis-like illness but EBV is classic.))

A Chinese man has cervical lymphadenopathy, headache and conductive deafness; suspected nasopharyngeal carcinoma. Pathogen?

• ((Epstein-Barr virus::☑️ Strongly linked to nasopharyngeal carcinoma in Southern Chinese populations.))
• ((HPV::Oropharyngeal SCC, not nasopharyngeal.))
• ((HHV-8::Kaposi's sarcoma.))
• ((HTLV-1::Adult T-cell leukaemia.))

A healthcare worker sustains a needlestick injury. Which infection carries the highest transmission risk?

• ((Hepatitis B::☑️ Up to 30% if source is HBeAg-positive.))
• ((Hepatitis C::~3%.))
• ((HIV::~0.3%.))
• ((HTLV-1::Very low risk.))
• ((Malaria::Negligible from needlestick.))

Which virus is associated with Kaposi's sarcoma?

• ((Human herpesvirus 8 (HHV-8)::☑️ Vascular tumour of skin/mucosa in AIDS.))
• ((HPV-16::Cervical and oropharyngeal SCC.))
• ((HTLV-1::Adult T-cell leukaemia.))
• ((EBV::Burkitt's, Hodgkin's, nasopharyngeal.))
• ((HPV-18::Cervical SCC.))

Which virus is associated with oropharyngeal carcinoma?

• ((HHV-8::Kaposi's sarcoma.))
• ((Human papillomavirus (HPV)::☑️ Particularly HPV-16; rising cause of tonsillar and base-of-tongue SCC.))
• ((HTLV-1::Adult T-cell leukaemia.))
• ((EBV::Nasopharyngeal, not oropharyngeal.))
• ((CMV::Post-transplant pneumonia.))

Best method to sterilise modern flexible endoscopes?

• ((Glutaraldehyde::☑️ High-level chemical disinfection suitable for delicate heat-sensitive optics.))
• ((Autoclave::Steam destroys flexible endoscope optics.))
• ((Ethylene oxide::Effective but slow, expensive — second choice if glutaraldehyde unavailable.))
• ((Chlorhexidine::Antiseptic, not a sterilant.))
• ((Gamma radiation::Used for disposables.))

An autoclave reaches only 100 °C. Which pathogen is most likely to survive?

• ((E. coli::Killed at 100 °C.))
• ((Clostridium species::☑️ Heat-resistant spores survive boiling; proper autoclave is 121 °C, 15 psi, 15 min.))
• ((Streptococcus::Killed at 100 °C.))
• ((Pseudomonas::Killed at 100 °C.))

Standard method to sterilise disposable plastic syringes?

• ((Boiling::Inadequate.))
• ((Autoclaving::Damages plastic.))
• ((Chemical sterilisation::Residues unacceptable.))
• ((Gamma irradiation::☑️ Industrial standard for single-use plastics.))
• ((Filtration::Used for heat-sensitive liquids.))

Revision summary

➡ Default skin/soft tissue and bone organism: S. aureus. Default cellulitis with lymphoedema: Strep pyogenes.

➡ Nec fasc: type I polymicrobial (Fournier's, diabetics); type II Group A Strep. Treat: meropenem + clindamycin (anti-toxin).

➡ Gas gangrene: C. perfringens, crepitus, dirty wound. Tetanus: C. tetani, rusty nail, trismus.

➡ Biliary/intra-abdominal sepsis: E. coli > Klebsiella > Enterococcus + anaerobes.

➡ UTI: E. coli (75%). Catheter UTI: Pseudomonas, Proteus, Candida. Staghorn: Proteus (urease, struvite).

➡ Pneumonia: CAP → S. pneumoniae; HAP → Pseudomonas/Klebsiella; aspiration → anaerobes; post-flu cavitation → S. aureus; alcoholic currant-jelly → Klebsiella.

➡ Osteomyelitis: S. aureus, except sickle cell → Salmonella, toddler → Kingella kingae.

➡ Endocarditis: native subacute = Strep viridans; IVDU = S. aureus (tricuspid → septic pulm emboli); prosthetic early = S. epidermidis. Mycotic aneurysm = S. aureus.

➡ Hospital acquired: C. difficile (oral vancomycin, spores survive alcohol gel), MRSA (vancomycin).

➡ Travel: typhoid (S. typhi, week 3 terminal ileal perforation); amoebic abscess (E. histolytica, anchovy-paste); hydatid (Echinococcus, sheep farmer, anaphylaxis if ruptured); Fasciola (watercress, eosinophilia).

➡ Bites: cat = Pasteurella; human = Eikenella; dog = Pasteurella ± Capnocytophaga (asplenic risk).

➡ Anthrax: painless black eschar, farmer.

➡ Transplant pneumonia: 1–6 months = CMV; > 6 months = EBV → PTLD.

➡ EBV cancers: Burkitt's, Hodgkin's, nasopharyngeal (Chinese), PTLD. HHV-8: Kaposi's. HPV: cervical, oropharyngeal. Schistosoma haematobium: bladder SCC.

➡ Needlestick risk: HBV 30% >> HCV 3% >> HIV 0.3%.

➡ Sterilisation: autoclave = standard instruments (121 °C); glutaraldehyde / ethylene oxide = endoscopes; gamma = disposable plastics; filtration = heat-sensitive liquids; Clostridium spores survive 100 °C boiling.