## **Core Concept**
The clinical presentation described suggests a case of **pertussis**, also known as whooping cough, which is a highly contagious respiratory illness caused by the bacterium *Bordetella pertussis*. The characteristic inspiratory whoop and cough in a non-immunized child are key features of this disease. Laboratory diagnosis of pertussis involves collecting a sample from the patient for culture, PCR, or other tests.

## **Why the Correct Answer is Right**
The correct answer involves collecting a **nasopharyngeal swab**. This is because *Bordetella pertussis* primarily colonizes the nasopharynx. A nasopharyngeal swab is the preferred sample for the diagnosis of pertussis, as it allows for the detection of the bacteria through PCR (polymerase chain reaction) or culture. PCR is particularly sensitive and is considered the gold standard for diagnosing pertussis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Blood sample - While blood samples can be used for some diagnostic tests, they are not the primary sample type used for the diagnosis of pertussis.
- **Option B:** Sputum sample - Sputum might be used in some respiratory infections, but it's not the preferred sample for diagnosing pertussis, especially in children who may not produce sputum.
- **Option D:** Throat swab - Although a throat swab could potentially be used, it is less sensitive compared to a nasopharyngeal swab for detecting *Bordetella pertussis*.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **whoop** in pertussis is often associated with a **post-tussive vomit**, and the disease is highly contagious, especially in the early stages. Vaccination status, as mentioned, plays a critical role in preventing the disease. The diagnosis is confirmed through **PCR** from a nasopharyngeal swab, making this an essential test in suspected cases.

## **Correct Answer:** . Nasopharyngeal swab