A 5-year-old child presents with fever, cervical lymphadenopathy, and membranous pharyngeal exudate. Culture on Loeffler serum medium grows Corynebacterium diphtheriae. The exotoxin's mechanism of action involves:

• A Activation of adenylyl cyclase via ADP-ribosylation of Gs protein
• B ADP-ribosylation of EF-2, halting protein synthesis ✓
• C Inhibition of acetylcholine release at neuromuscular junctions
• D Irreversible binding to 50S ribosomal subunit

Explanation

Diphtheria toxin (a phage-encoded A-B toxin) has a catalytic A subunit that ADP-ribosylates elongation factor 2 (EF-2), which is required for translocation of the ribosome along mRNA. This irreversibly halts eukaryotic protein synthesis, causing cell death. This mechanism underlies myocarditis (a leading cause of death) and demyelinating neuropathy in diphtheria. Botulinum toxin inhibits acetylcholine release; cholera toxin and pertussis toxin act via Gs-protein ADP ribosylation.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.