A 28-year-old woman from rural Maharashtra presents with a 3-week history of migratory polyarthralgia, fever, and a new cardiac murmur (Carey Coombs murmur). ESR is 85 mm/hr and CRP is elevated. She gives a history of pharyngitis 4 weeks ago. Regarding the pathogenesis and sequelae of Streptococcus pyogenes infection, all of the following are true EXCEPT:

Explanation

## Post-Streptococcal Sequelae and Pathogenic Mechanisms ### Clinical Context This patient presents with classic acute rheumatic fever (ARF) — migratory polyarthralgia, carditis (Carey Coombs murmur), elevated inflammatory markers, and a clear temporal link to pharyngitis 4 weeks prior. The question tests understanding of pathogenic mechanisms in GAS sequelae. ### Analysis of Each Statement **Statement 1 (TRUE): Molecular Mimicry in ARF** **Key Point:** Acute rheumatic fever is driven by cross-reactive antibodies. M protein shares epitopes with human myosin (cardiac), tropomyosin, and keratin. This molecular mimicry triggers autoimmune damage. **High-Yield:** The latency period (2–3 weeks) allows time for antibody maturation and cross-reactive immune responses to develop. **Statement 2 (TRUE): Post-Streptococcal Glomerulonephritis (PSGN)** **Key Point:** PSGN is mediated by immune complex deposition in the glomerular basement membrane (GBM), typically in a "starry sky" pattern on immunofluorescence (IgG and C3). **Clinical Pearl:** PSGN typically occurs 1–2 weeks after skin infection (impetigo) or 2–3 weeks after pharyngitis. Unlike ARF, it does NOT recur with subsequent GAS infections. **Statement 3 (TRUE): Streptococcal Pyrogenic Exotoxins (SPEs)** **Key Point:** SPEs (SPE-A, SPE-B, SPE-C, etc.) are superantigens that bypass normal antigen processing. They directly cross-link MHC class II molecules on antigen-presenting cells with T-cell receptors (TCR), causing massive T-cell activation (up to 20% of T cells vs. 0.01% in conventional antigen presentation). **High-Yield:** This superantigen mechanism is responsible for the systemic inflammatory response in invasive GAS disease and scarlet fever. **Statement 4 (FALSE): Scarlet Fever Rash Mechanism** **Key Point:** Scarlet fever is caused by lysogenic phages carrying genes for erythrogenic toxin (SPE-A, SPE-B, or SPE-C). However, the rash is **NOT an allergic reaction** — it is a **direct toxic effect** of the exotoxin on capillary endothelium and a manifestation of the superantigen-driven systemic inflammatory response. **Warning:** The rash in scarlet fever is **toxin-mediated**, not IgE-mediated hypersensitivity. This is a common misconception. **Mnemonic:** **SPE = Superantigen** (direct T-cell activation); **Rash = Toxin effect** (not allergy). ### Comparison Table: GAS Sequelae | Feature | Acute Rheumatic Fever (ARF) | Post-Streptococcal GN (PSGN) | Scarlet Fever | | --- | --- | --- | --- | | **Pathogenesis** | Molecular mimicry (cross-reactive antibodies) | Immune complex deposition | Superantigen + exotoxin | | **Latency** | 2–3 weeks (pharyngitis) | 1–2 weeks (skin) or 2–3 weeks (throat) | Same as acute infection | | **Rash type** | None | None | Sandpaper texture (toxin-mediated) | | **Recurrence risk** | High with repeat GAS infection | Low (type-specific immunity) | Depends on toxin-producing strain | | **Cardiac involvement** | Carditis (Carey Coombs murmur) | Rare | None | **Clinical Pearl:** The Carey Coombs murmur (mid-diastolic, heard at the apex) is a sign of acute mitral regurgitation due to valve inflammation in ARF, not stenosis.