A 3-year-old child in Tamil Nadu develops sudden high fever, sore throat, and a fine, diffuse erythematous rash with circumoral pallor and a 'sandpaper' texture. The tongue has a white coating with enlarged red papillae. What is the virulence mechanism of the causative toxin?
- A ADP-ribosylation of EF-2 blocking protein synthesis
- B Inhibits acetylcholine release at neuromuscular junctions
- C Superantigen non-specifically cross-linking MHC II to Vβ regions of TCR ✓
- D Cleaves fibronectin to facilitate tissue invasion
Explanation
Scarlet fever is caused by group A Streptococcus (S. pyogenes) that produces pyrogenic exotoxins (SPE-A, SPE-C, SSA, SMEZ). These are superantigens that non-specifically cross-link MHC class II molecules on APCs with the Vβ region of T-cell receptors, bypassing normal antigen specificity. This activates up to 20–30% of T cells (vs 0.001% for normal antigens), causing a massive cytokine storm (TNF, IL-1, IL-2) responsible for the fever and rash. ADP-ribosylation of EF-2 is diphtheria toxin's mechanism. Inhibition of acetylcholine is tetanus/botulinum toxin. Fibronectin cleavage is a streptokinase/hyaluronidase role.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.