A 4-month-old unimmunized infant from rural India presents with a 3-week history of paroxysmal coughing spells. The mother reports that after each bout of rapid coughs, the infant takes a deep, forced inspiration that produces a characteristic high-pitched sound before the next coughing fit begins. The structure marked **C** in the diagram represents this clinical finding. Which of the following best explains the mechanism underlying this sound?

Explanation

## Why "Forced inspiration through a partially closed glottis after a series of coughs, characteristic of the paroxysmal phase of pertussis" is right The inspiratory "whoop" sound marked **C** is the pathognomonic clinical sign of pertussis in the paroxysmal phase. It occurs when the infant takes a forced, gasping inspiration through a partially closed glottis immediately after a paroxysm of rapid coughs. This is a hallmark feature of Bordetella pertussis infection and is directly caused by the mechanics of the coughing reflex and glottic closure during the recovery phase of the paroxysm. The sound is absent in infants <6 months who lack the inspiratory effort to generate it, making its presence a key diagnostic clue in older infants and children (Nelson 21e; Harrison 21e Ch 169). ## Why each distractor is wrong - **Stridor from laryngeal edema caused by Bordetella pertussis toxin-mediated inflammation**: While pertussis toxin does cause systemic effects, stridor is a continuous high-pitched breathing sound, not the characteristic whoop that occurs specifically after coughing paroxysms. Stridor is not a typical feature of pertussis. - **Wheezing from bronchospasm triggered by pertussis toxin-induced mucus hypersecretion**: Wheezing is a lower-pitched, continuous sound from small airway obstruction and is not the defining feature of pertussis. The whoop is distinct and occurs at a specific point in the coughing cycle, not from bronchospasm. - **Crackles from secondary bacterial pneumonia superimposed on pertussis infection**: While secondary pneumonia is a common and serious complication of pertussis, crackles are fine, discontinuous sounds heard on auscultation during inspiration. They do not represent the characteristic whoop sound that defines the paroxysmal phase. **High-Yield:** The inspiratory whoop is the pathognomonic sign of pertussis in the paroxysmal phase; its absence in infants <6 months (who present instead with apnea and cyanosis) is a critical diagnostic clue that delays recognition and increases mortality risk. [cite: Nelson 21e; Harrison 21e Ch 169]