A 6-week-old infant presents with 3 days of coryza followed by paroxysmal cough with post-tussive vomiting and cyanosis during coughing episodes. No 'whoop' is heard due to the infant's age. Lymphocyte count is 24,000/μL. Nasopharyngeal PCR is positive for Bordetella pertussis. The pathogenic toxin responsible for the characteristic lymphocytosis is:
- A Adenylate cyclase toxin (ACT)
- B Dermonecrotic toxin (DNT)
- C Pertussis toxin (PT) — causes lymphocytosis by blocking lymphocyte recirculation ✓
- D Tracheal cytotoxin (TCT)
Explanation
Pertussis toxin (PT) is the key exotoxin of Bordetella pertussis responsible for the systemic manifestations of whooping cough. PT ADP-ribosylates the Gi (inhibitory) alpha-subunit of heterotrimeric G proteins, blocking signaling for lymphocyte recirculation out of the blood. This causes a characteristic marked lymphocytosis (predominantly lymphocytes, not neutrophils) — counts may reach 50,000–100,000/μL in severe infant pertussis. Adenylate cyclase toxin raises intracellular cAMP and impairs phagocyte function. Tracheal cytotoxin damages ciliated respiratory epithelium. DNT causes local tissue necrosis.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.