**Core Concept:** Whooping cough, also known as pertussis, is a respiratory infection caused by the bacterium Bordetella pertussis. Clinical features include prolonged, paroxysmal coughing fits followed by a characteristic inspiratory "whoop" sound due to hyperinflation of the lungs after the coughing fit subsides. In immunocompromised patients like this child, the severity and duration of the symptoms are often prolonged.

**Why the Correct Answer is Right:** The correct answer, C, refers to the clinical sample required for diagnosing pertussis. Investigations like blood tests, radiographs, and cultures can provide useful information, but the gold standard for diagnosis is the detection of Bordetella pertussis antigens or DNA in the respiratory samples. In this case, considering the child's symptoms and lack of immunity, a throat swab or nasopharyngeal aspirate (NPA) is the appropriate sample to be taken.

**Why Each Wrong Option is Incorrect:**

A. Urine sample (Urea and electrolyte imbalance) is not relevant to diagnose pertussis.

B. Blood sample (serology) is essential for evaluating immune response but does not directly diagnose Bordetella pertussis infection.

D. Chest X-ray (imaging) helps in detecting complications but not directly diagnosing the causative agent.

**Why the Correct Answer is Right (Repeated)**

Throat swab/NPA is the right sample to diagnose pertussis because it allows the detection of Bordetella pertussis antigens or DNA in the respiratory samples. This helps identify the causative agent responsible for the child's symptoms and informs appropriate treatment decisions.

**Clinical Pearl:** In children presenting with cough and whooping cough symptoms, promptly obtaining an appropriate respiratory sample for diagnosis is crucial. Delayed diagnosis can lead to prolonged illness and complications. Early initiation of appropriate antibiotics can reduce the severity and duration of the disease and prevent complications like pneumonia, encephalopathy, and bronchopneumonia.