**Question:** A child with complaints of cough. Characteristic 'inspiratory whoop. Sample for investigation is –

A. Sputum
B. Blood sample
C. Chest X-ray
D. Bronchoalveolar lavage (BAL)

**Core Concept:** Inspiratory whoop is a characteristic sound heard during chest auscultation in children with certain respiratory infections, particularly in cases involving the lower airways.

**Why the Correct Answer is Right:** In the context of a child with cough and inspiratory whoop, the most appropriate sample for investigation would be a **Chest X-ray** (Option C). A chest X-ray helps to visualize lung parenchyma, airways, and pleural space, which is essential for identifying radiological abnormalities associated with lower respiratory tract infections.

**Why Each Wrong Option is Incorrect:**

**A. Sputum:** While sputum sampling can be useful in certain cases, it is generally acquired from adults and not children. Furthermore, it may not accurately represent the entire respiratory tract infection, particularly in the young child.

**B. Blood sample:** A blood sample is typically obtained to evaluate systemic infections or underlying immunological disorders. In the context of a child with cough and inspiratory whoop, it is not the primary sample of choice for diagnosing lower respiratory tract infections.

**D. Bronchoalveolar lavage (BAL):** BAL is a more invasive procedure that involves flushing the lower respiratory tract with a sterile solution to collect samples from the alveoli. This procedure is generally reserved for more indepth diagnostic investigations and is not the first-line sample for a child with cough and inspiratory whoop.

**Clinical Pearl:** In a child with cough and inspiratory whoop, a chest X-ray is a non-invasive, readily available, and reliable tool to evaluate lower respiratory tract infections. It aids in identifying the site of infection, assessing lung parenchyma, and detecting pleural effusion or pneumothorax, which are common complications in lower respiratory tract infections.

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**Correct Answer: Chest X-ray (C)**

In the context of a child with cough and inspiratory whoop, a chest X-ray is the most appropriate sample for investigation as it offers valuable information regarding lower respiratory tract infections. Chest X-ray aids in visualizing lung parenchyma, airways, and pleural space, which is essential for identifying the site of infection, assessing lung parenchyma, and detecting pleural effusion or pneumothorax, which are common complications in lower respiratory tract infections.