A 9-month-old with a cough and a whoop sound during inhalation likely has which condition?

The presentation of a cough accompanied by a whooping sound during inhalation is characteristic of pertussis, commonly known as whooping cough. This condition is caused by the bacterium Bordetella pertussis and is particularly notable for its distinctive coughing fits that can end in a high-pitched "whoop" as the individual inhales sharply after a coughing episode.

Croup typically presents with a “barking” cough and is often associated with stridor and hoarseness, generally due to viral infection causing upper airway obstruction. Bronchiolitis, usually seen in younger infants, manifests with wheezing and respiratory distress as a result of lower airway obstruction. Pneumonia can present with a cough but usually involves fever, potentially productive cough, and respiratory distress without the characteristic whoop.

Given the specific symptom of a whooping sound during inhalation, pertussis is clearly the most accurate diagnosis in this scenario.