A 4-year-old boy from rural Maharashtra presents with a 3-week history of severe paroxysmal cough fits, followed by an inspiratory "whoop" sound, post-tussive vomiting, and exhaustion. On examination, the structure marked **A** in the diagram is noted. A CBC shows WBC count of 65,000/μL with lymphocytic predominance. Which of the following is the most appropriate first-line antimicrobial agent for this patient?

## Why Azithromycin is right The clinical presentation of violent paroxysmal coughing with inspiratory whoop, post-tussive vomiting, and the characteristic finding of subconjunctival hemorrhage (marked **A**) from increased intrathoracic pressure during cough paroxysms is pathognomonic for pertussis caused by *Bordetella pertussis*. The marked lymphocytosis (65,000/μL) further supports this diagnosis. Azithromycin is the first-line macrolide antibiotic for pertussis in India and globally, given as a 5-day course. It is preferred over erythromycin due to better tolerability and lack of association with infantile hypertrophic pyloric stenosis (Nelson 21e Ch 224; Park 26e). ## Why each distractor is wrong - **Erythromycin**: Although historically used for pertussis, erythromycin is no longer first-line. It carries a significant risk of infantile hypertrophic pyloric stenosis, particularly in infants <1 month of age, and has inferior tolerability compared to azithromycin. - **Trimethoprim-sulfamethoxazole**: This is a second-line agent reserved for patients with documented macrolide allergy or resistance. It is not the initial choice in a macrolide-naive patient. - **Ceftriaxone**: Third-generation cephalosporins are not effective against *Bordetella pertussis* and are not indicated for pertussis treatment. They may be used for secondary bacterial pneumonia if it develops. **High-Yield:** Azithromycin 5 days is first-line for pertussis; erythromycin is avoided due to pyloric stenosis risk in infants <1 month. [cite: Nelson 21e Ch 224; Park 26e]