In which condition is the "whooping" sound primarily noted?

The presence of the characteristic "whooping" sound is primarily associated with pertussis, also known as whooping cough. This infectious disease is caused by the bacterium Bordetella pertussis and is characterized by severe coughing spells that can culminate in a high-pitched whoop during inhalation following a paroxysm of coughing.

The whooping sound is indicative of the airflow restriction caused by inflammation and mucus accumulation in the airways, which leads to difficulty in breathing. In infants and young children, this sound is particularly notable and can be alarming, as the coughing fits may lead to a significant reduction in oxygen intake and potential respiratory distress.

While asthma, croup, and pneumonia can also cause respiratory distress and varying cough sounds, they do not typically present with the classic whooping sound associated with pertussis. Asthma may lead to wheezing, croup often results in a "barking" cough, and pneumonia typically involves a productive cough along with other signs like fever and difficulty breathing. Thus, the hallmark whooping sound distinguishing pertussis makes it the correct answer in this context.