A fully vaccinated 2-month-old infant develops increasing paroxysms of cough ending in vomiting. Total leukocyte count is 32,000 per cubic mm with 80 percent mature small lymphocytes. Chest radiograph is unremarkable. Which organism is the most likely cause?
- A Chlamydia trachomatis
- B Adenovirus
- C Bordetella pertussis ✓
- D Respiratory syncytial virus
Correct answer: C. Bordetella pertussis
Explanation
Marked absolute lymphocytosis out of proportion to clinical findings is the classic laboratory signature of pertussis, produced by pertussis toxin acting on lymphocytes. Infants are vulnerable before completing primary vaccination, so full protection of a 2-month-old is impossible despite maternal Td vaccination. Chlamydia trachomatis pneumonia in infancy features staccato cough with eosinophilia, not lymphocytosis, and RSV causes bronchiolitis with wheeze rather than pure paroxysmal cough with lymphocytosis.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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