**Core Concept**
The characteristic inspiratory whoop in a child with cough is suggestive of pertussis, a highly contagious bacterial infection caused by *Bordetella pertussis*. Diagnosing pertussis is crucial for early intervention and prevention of transmission.

**Why the Correct Answer is Right**
Nasopharyngeal swab (NPS) is the preferred specimen for diagnosing pertussis. This is because *B. pertussis* colonizes the nasopharyngeal mucosa, making NPS more likely to yield a positive culture. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend NPS for pertussis diagnosis. NPS is also less invasive than tracheal aspiration and more practical than sputum culture.

**Why Each Wrong Option is Incorrect**
**Option A:** Tracheal aspiration is not the preferred method for diagnosing pertussis, as it is more invasive and not commonly used for this purpose.
**Option B:** Sputum culture is not ideal for pertussis diagnosis, as the bacteria may not be present in sputum and the culture may be contaminated with other pathogens.

**Clinical Pearl / High-Yield Fact**
In cases of suspected pertussis, it's essential to consider the vaccination status of the patient and their close contacts, as vaccination is the most effective measure for preventing the disease.

**Correct Answer Line**
✓ Correct Answer: A. Nasopharyngeal swab