A 6-year-old unvaccinated child from rural India presents with a 2-week history of paroxysmal cough with inspiratory stridor and post-tussive vomiting. Another child in the same village has a sore throat, low-grade fever, and a thick grayish-white membrane over the tonsils with cervical lymphadenopathy. Which single clinical feature best distinguishes pertussis from diphtheria?

Explanation

## Distinguishing Pertussis from Diphtheria ### Key Clinical Discriminator **Key Point:** The **'whoop'** — a characteristic high-pitched inspiratory sound heard at the end of a paroxysmal cough bout — is pathognomonic for pertussis and virtually absent in diphtheria. ### Comparative Table | Feature | Pertussis | Diphtheria | | --- | --- | --- | | **Cough character** | Paroxysmal, 15–50 coughs/bout; ends with 'whoop' | Mild, non-paroxysmal | | **Membrane** | None | Thick, grayish-white, adherent, on tonsils/pharynx | | **Systemic toxemia** | Minimal; mainly respiratory | Prominent; myocarditis, neuropathy, toxic shock | | **Fever** | Absent or minimal (afebrile) | Low-grade, early | | **Cough onset** | Insidious; 1–2 weeks of catarrhal stage | Acute onset | | **Causative agent** | *Bordetella pertussis* (respiratory epithelium) | *Corynebacterium diphtheriae* (toxin-mediated) | ### Why 'Whoop' is the Gold Standard Discriminator **High-Yield:** The whoop occurs because: 1. Pertussis toxin causes ciliary paralysis and mucus plugging. 2. Paroxysmal cough expels the plug; the sudden reopening of the glottis against a narrowed airway creates the characteristic high-pitched stridor. 3. This is **unique to pertussis** and rarely seen in diphtheria (which causes mild, non-paroxysmal cough). **Clinical Pearl:** Not all pertussis cases produce an audible whoop — infants <6 months and vaccinated individuals may cough without the classic sound. However, when present, it is virtually diagnostic of pertussis. ### Why Other Features Are Not Discriminators - **Pseudomembrane:** Pathognomonic for diphtheria, not pertussis — so it distinguishes diphtheria FROM pertussis, not the reverse. - **Systemic toxemia:** Hallmark of diphtheria (toxin-mediated), not pertussis — again, distinguishes diphtheria. - **Fever pattern:** Both can present with fever; pertussis is often afebrile, but fever is not a discriminator when present in both. **Mnemonic:** **WHOOP = Whooping cough = Pertussis** — the sound is the signature finding.