A 6-year-old unvaccinated child from rural India presents with a 2-week history of paroxysmal cough with inspiratory 'whoop' and post-tussive vomiting. Blood culture and nasopharyngeal culture are negative. Which single clinical feature most reliably distinguishes pertussis from diphtheria in this child?

Explanation

## Distinguishing Pertussis from Diphtheria ### The Key Discriminator **Key Point:** The paroxysmal cough with inspiratory 'whoop' and post-tussive vomiting is the pathognomonic hallmark of pertussis and is virtually absent in diphtheria. This feature alone can reliably differentiate the two conditions at the bedside. ### Comparative Clinical Features | Feature | Pertussis | Diphtheria | |---------|-----------|----------| | **Cough character** | Paroxysmal, with 'whoop' and post-tussive vomiting | Non-paroxysmal, often mild | | **Pseudomembrane** | Absent | Present (hallmark finding) | | **Onset fever** | Minimal or absent ("cough without fever") | High fever, systemic toxicity | | **Lymphadenopathy** | Generalized, non-tender | Cervical ("bull neck" appearance) | | **Complications** | Apnea, seizures, encephalopathy (pertussis toxin) | Myocarditis, neuropathy (diphtheria toxin) | | **Leukocytosis** | Marked (often >20,000/µL) | Mild to moderate | ### Why the 'Whoop' Matters **High-Yield:** The 'whoop' is produced by rapid inspiration against a partially closed glottis following a paroxysm of coughing. This occurs only in pertussis due to the specific pathophysiology of *Bordetella pertussis* infection and tracheal inflammation. Diphtheria causes a membranous obstruction but does NOT produce this characteristic sound. **Clinical Pearl:** In pertussis, the cough is so severe that children often vomit after coughing fits, leading to the mnemonic "100-day cough" (lasting weeks to months). Diphtheria, by contrast, presents with a sore throat, low-grade fever, and a mild cough — the disease is defined by the membrane, not the cough. ### Diagnostic Confirmation **Mnemonic:** **WHOOP** = **W**hooping cough (pertussis), **H**igh paroxysms, **O**ften post-tussive vomiting, **O**ver weeks, **P**athognomonic sign. In this case, the negative cultures do not exclude either diagnosis — culture sensitivity for pertussis is low after the first 2 weeks (catarrhal stage), and diphtheria diagnosis relies on clinical findings + toxin detection, not culture alone.