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

**Core Concept:**
The presence of a characteristic inspiratory whoop, a high-pitched, short, and sharp sound, is a clinical sign associated with certain respiratory diseases in children, particularly in cases of whooping cough (pertussis). This question is testing our understanding of the appropriate sample collection for further investigation to confirm the diagnosis.

**Why the Correct Answer is Right:**
In this scenario, the correct sample for investigation is a **sputum** specimen, specifically the induced sputum. The whooping cough is a respiratory illness caused by the bacterium Bordetella pertussis, which primarily affects the respiratory tract. To confirm the diagnosis, we require a sample that contains the causative agent and allows for laboratory testing, such as culture, serology, or PCR.

**Why Each Wrong Option is Incorrect:**

A. **Blood:** Although blood tests can provide general clues about the patient's condition, they are not suitable for diagnosing whooping cough directly as the bacterium is primarily located in the respiratory tract.

B. **Urine:** Urine samples are not suitable for diagnosing respiratory infections, including whooping cough, as the bacterium is not present in the urine.

C. **Blood culture:** While blood cultures can detect Bordetella pertussis in severe cases, they are not as specific or sensitive as sputum culture for diagnosing whooping cough.

D. **Sputum:** Sputum culture is less specific than induced sputum culture, which involves the collection of sputum that is induced from the lower respiratory tract using techniques such as nebulization or bronchoscopy. This increases the chances of detecting Bordetella pertussis and provides a more accurate diagnosis of whooping cough.

**Clinical Pearl:**
Induced sputum culture is a valuable diagnostic tool for whooping cough in clinical practice. Induced sputum collection using bronchoscopy or nebulization is a more reliable method compared to obtaining spontaneous sputum due to the higher concentration of Bordetella pertussis in the lower respiratory tract. By collecting induced sputum, we can accurately diagnose whooping cough and initiate appropriate treatment promptly, which is essential for the child's well-being.