Streptococcus pyogenes

1. "Expert practice tip 3: β-haemolytic streptococci are sensitive to penicillin" — directly from GC 105 slides ^[1]. This is a recurring exam principle: GAS has no reported penicillin resistance. Penicillin V is first-line for pharyngitis; penicillin G for invasive disease.

2. GC 105 lists GAS clinical spectrum as: pharyngitis, scarlet fever, impetigo, cellulitis, necrotizing fasciitis, TSS, and post-infectious syndromes (ARF, PSGN) ^[1]. Expect MCQs or SAQs listing "5 non-cardiac diseases caused by GAS" — pharyngitis, scarlet fever, TSS, cellulitis, PSGN is a classic five ^[10].

3. Superantigen mechanism: toxin binds to part of TCR shared by many T cells → massive T cell proliferation and cytokine release ^[5]. This mechanism underlies both scarlet fever and streptococcal TSS and is a favourite MCQ discriminator.

4. ARF follows streptococcal pharyngitis ONLY — impetigo/pyoderma does NOT cause ARF but CAN cause PSGN ^[4]. This is an extremely high-yield exam discriminator tested repeatedly.

5. PSGN timing: 1–3 weeks after pharyngitis; 3–6 weeks after skin infection. Lab hallmark: ↓C3 with normal C4 + ↑ASO titre. Subepithelial humps ("starry sky" granular IgG/C3 on IF) ^[7][8].

6. NF caused by S. pyogenes: pain out of proportion to clinical findings, haemorrhagic bullae, dishwater discharge; LRINEC score > 8 = high risk; management = aggressive debridement + IV antibiotics ^[6]. Exam favourite in surgery papers.

• GAS vs GBS: GAS = bacitracin-sensitive, β-haemolytic, pharyngitis/skin/ARF/PSGN. GBS = bacitracin-resistant, β-haemolytic, neonatal sepsis/meningitis, peripartum infections, UTI.
• Scarlet fever vs Kawasaki disease: Both present in children with fever and rash. Scarlet fever has sandpaper rash on day 2, circumoral pallor, strawberry tongue, Pastia's lines, and responds to penicillin. Kawasaki requires ≥5 days fever, bilateral non-exudative conjunctivitis, extremity changes, and needs IVIG + aspirin.
• Streptococcal TSS vs Staphylococcal TSS: Strep TSS requires GAS isolation from normally sterile site + hypotension + ≥2 organ criteria. Staph TSS is toxin-mediated (TSST-1), associated with tampon use/wound packing, and does not require positive sterile-site culture.
• PSGN complement pattern: ↓C3 with normal C4 (alternative pathway activation). If C3 AND C4 both low → think lupus nephritis or MPGN. If C3 remains low > 8 weeks → suspect lupus nephritis or MPGN, not PSGN ^[8].
• ASO titre pitfall: ASO may be negative in skin-strain PSGN; anti-DNase B is more reliable for post-skin-infection PSGN ^[8].

2025 Fourth Summative MCQ — EMQ Section IV "Use of Antibiotics", Q20 ^[9]:

"Treatment of group A streptococcal pharyngitis." Options included: A. Amoxicillin-clavulanic acid, B. Cefazolin, C. Cefotaxime, D. Ceftaroline, E. Ceftazidime, F. Cloxacillin, G. Gentamicin, H. Nitrofurantoin, I. Penicillin V, J. Rifampicin.

Correct answer: I. Penicillin V. Rationale: GAS pharyngitis first-line is oral penicillin V. GAS remains universally penicillin-sensitive. Cloxacillin (anti-staphylococcal) and cephalosporins are not first-line. This directly tests the GC 105 "expert practice tip 3" principle ^[1].

2021 Fourth Summative MCQ — EMQ Section V "Investigating a Child with Fever and Rash", Q19 ^[11]:

"A 4-year-old girl presented with a low-grade fever, sore throat and a generalised, blanchable, erythematous, maculopapular rash. Her blood count revealed presence of atypical lymphocytes."

Correct answer: D. Monospot test. Rationale: Atypical lymphocytes + pharyngitis + maculopapular rash → EBV infectious mononucleosis, NOT scarlet fever. This is a discriminator trap: scarlet fever rash has sandpaper texture and typically circumoral pallor, and would NOT show atypical lymphocytes. The monospot differentiates EBV from GAS pharyngitis.

(Note: While Q19 above tests EBV, the EMQ stem set also includes options for ASO titre and throat swab for bacterial culture — examiners expected students to differentiate GAS-related investigations from EBV-related ones in this question set.)

No other past paper questions specifically featuring S. pyogenes as the primary organism were identified in the indexed past paper context. Questions on ARF/PSGN/NF may appear in SAQ or minicase format but the specific stems were not retrievable from the indexed text provided.