An unimmunized 8-week-old infant presents with 10 days of low-grade fever and mild cough, followed by increasingly frequent episodes of sudden repetitive coughing followed by an inspiratory gasp. Post-tussive vomiting occurs. Total leukocyte count is 28,000/mm3 with 70% small mature lymphocytes. Which organism is responsible?

• A Mycoplasma pneumoniae
• B Adenovirus
• C Bordetella pertussis ✓
• D Haemophilus influenzae type b

Explanation

The catarrhal stage followed by paroxysmal cough ending in inspiratory whoop with post-tussive vomiting is typical of pertussis. Bordetella pertussis causes marked absolute lymphocytosis due to pertussis toxin-mediated lymphocyte release, a key discriminator from other causes of cough where neutrophilia predominates. Infants under 3 months are at highest risk before primary vaccination, and apnea may replace the whoop in young infants.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.