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Q1: Mechanism and recognition (3 min)
Examiner: A 23-year-old menstruating woman using tampons presents with fever, confusion, hypotension and a diffuse sunburn-like rash with a 'strawberry tongue'. A retained tampon is found. Walk me through the mechanism and why this is toxic shock syndrome.
Expected answer:
- TSS is an acute, toxin-mediated (superantigen) multisystem illness. The toxin is TSST-1 from Staphylococcus aureus colonising the vagina and absorbed across the vaginal mucosa.
- Normal antigen presentation: APC processes antigen into peptide fragments, loads them into the groove of MHC class II; only T-cells whose TCR CDR3 region recognises that peptide-MHC complex are activated (~0.01 percent of all T-cells).
- Superantigen bypass: TSST-1 binds DIRECTLY to the OUTER side surface of MHC class II (NOT in the peptide groove) AND to the V-beta region of the TCR (NOT the antigen-specific CDR3), bridging APC and T-cell without antigen processing or specificity.
- Result: 20 to 30 percent of ALL T-cells activated simultaneously -> cytokine storm (IL-1, IL-2, TNF-alpha, IFN-gamma) -> capillary leak, vasodilation, hypotension, multi-organ failure.
- The clinical picture (fever over 38.9 deg C + diffuse blanching rash + hypotension + multi-organ involvement) meets the CDC criteria. Organism isolation is NOT required — staphylococcal TSS is typically NOT bacteraemic (blood cultures negative).
Follow-up: Why is the tampon the source? High-absorbency tampons bind magnesium (derepressing TSST-1 production) and create an oxygenated, protein-rich, neutral-to-alkaline environment ideal for TSST-1; the vaginal mucosa absorbs the preformed toxin.
Q2: CDC criteria and the staph vs strep distinction (3 min)
Examiner: Reproduce the CDC case definition for staphylococcal TSS, and distinguish it from streptococcal TSS.
Expected answer — CDC staphylococcal TSS (all 6 required):
- Fever over 38.9 deg C.
- Diffuse macular rash.
- Desquamation 1 to 2 weeks later (especially palms/soles).
- Hypotension (systolic at or under 90 mmHg adults, or orthostatic drop).
- Multi-organ involvement — 3 or more of: GI (vomiting/diarrhoea at onset), muscular (severe myalgia or CPK over 2x ULN), mucous membrane (vaginal/oropharyngeal/conjunctival hyperaemia), renal (BUN/Cr over 2x ULN or sterile pyuria), hepatic (bilirubin/AST/ALT over 2x ULN), haematologic (platelets under 100,000/microL), CNS (disorientation without focal signs).
- Negative cultures/serology for other causes (blood may be positive but is NOT required).
Staphylococcal vs streptococcal distinction:
- Staph: typically NOT bacteraemic (blood cultures negative), organism isolation NOT required, source is a colonised focus (tampon/wound), mortality 3 to 5 percent.
- Strep (STSS): Group A strep isolated from a sterile site (bacteraemia in ~60 percent), invasive soft-tissue infection (necrotising fasciitis), requires only 2 or more organ systems, mortality 30 to 60 percent.
Q3: Management ladder with doses (3 min)
Examiner: How do you manage this patient, with doses?
Expected answer — stepwise (R-A-S-C-I-V):
- Resuscitate — ABCDE; high-flow oxygen; two large-bore cannulae; IV crystalloid 10 to 20 mL/kg boluses repeated, titrated to MAP at least 65 mmHg and urine output at least 0.5 mL/kg/h.
- Antibiotics within 1 hour — clindamycin 600 to 900 mg IV q8h (anti-toxin) + flucloxacillin 2 g IV q6h (MSSA) + vancomycin 15 to 20 mg/kg IV q8 to 12h (MRSA) + benzylpenicillin 2.4 g IV q4 to 6h (GAS). Duration typically 10 to 14 days.
- Source control — REMOVE the tampon immediately (can abort the syndrome); for STSS, urgent surgical debridement of necrotising fasciitis (life-saving; do NOT delay to stabilise).
- Clindamycin is the cornerstone anti-toxin — suppresses TSST-1/Spe synthesis by ~90 percent; penicillin alone may INCREASE toxin release by lysis.
- IVIG 1 to 2 g/kg IV single infusion for severe/refractory disease — provides anti-TSST-1 antibodies, neutralises circulating superantigen.
- Ventilation, vasopressors (noradrenaline), VTE prophylaxis, vital-organ support (ICU) — mechanical ventilation for ARDS, renal replacement therapy for AKI.
Follow-up: Why is clindamycin mandatory, not just penicillin? Clindamycin suppresses TSST-1 protein synthesis (the anti-toxin effect) and is unaffected by the Eagle effect; penicillin alone may transiently INCREASE toxin release by lysing bacteria.
Q4: The streptococcal variant and surgery (2 min)
Examiner: How does streptococcal TSS differ, and what is the surgical role?
Expected answer:
- STSS = Group A strep (S. pyogenes, SpeA/SpeC) invasive soft-tissue infection — necrotising fasciitis in ~50 percent, myositis, cellulitis, pneumonia, postpartum sepsis. Bacteraemia in ~60 percent (vs uncommon in staphylococcal TSS).
- Mortality 30 to 60 percent (vs 3 to 5 percent for staphylococcal TSS).
- Management: benzylpenicillin 2.4 g IV q4 to 6h + clindamycin 900 mg IV q8h + IVIG 1 to 2 g/kg + URGENT surgical debridement.
- Surgery is life-saving: explore within hours, debride all necrotic tissue to bleeding viable tissue, re-look at 24 to 48 h, amputate if limb gangrene. Do NOT delay surgery to 'stabilise' the patient — the patient stabilises only after source control.
- LRINEC score (CRP, WBC, Hb, Na, creatinine, glucose): a score of 6 or more indicates high risk of necrotising fasciitis — but LRINEC is insensitive; a high clinical suspicion (pain out of proportion) overrides a 'low' score.
Q5: Pitfalls, recurrence and counselling (2 min)
Expected answer:
- The 'benign wound' trap — post-surgical and postpartum TSS frequently present with a wound that looks clean and closed; the diagnosis is the systemic picture, NOT the wound appearance.
- Drug-exam trap — penicillin/oxacillin alone may INCREASE toxin release by lysis; ALWAYS add clindamycin (anti-toxin); linezolid is the MRSA alternative (also suppresses toxin).
- Desquamation of palms and soles 1 to 2 weeks after onset is PATHOGNOMONIC — confirms a retrospective TSS diagnosis.
- Recurrence — ~10 percent of adults lack anti-TSST-1 antibody and are susceptible; ~30 percent recurrence for menstrual TSS without counselling. IVIG supplies these antibodies.
- Counselling — lowest-absorbency tampon, change every 4 to 8 hours, avoid overnight use, no tampons after an episode of menstrual TSS; avoid barrier contraceptives.
- Mortality discriminator: staphylococcal TSS 3 to 5 percent vs streptococcal TSS 30 to 60 percent — an exam favourite.