## **Core Concept**
The question involves interpreting a chest X-ray finding of bilateral lung infiltrates and determining the next appropriate investigation. This scenario often presents in patients with respiratory symptoms and can be indicative of infections, inflammatory conditions, or other lung pathologies. The next step in management typically involves further diagnostic testing to narrow down the cause.

## **Why the Correct Answer is Right**
The correct answer, **High-Resolution Computed Tomography (HRCT) of the chest**, is the most appropriate next investigation for several reasons. HRCT provides detailed images of the lung parenchyma, allowing for better characterization of the extent and distribution of lung infiltrates. It can help differentiate between various causes such as pneumonia, interstitial lung disease, and pulmonary edema. HRCT is particularly useful for guiding further management and potentially obviating the need for more invasive procedures.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **Complete Blood Count (CBC)** can provide valuable information on the patient's inflammatory and immune status, it does not directly assess the lung parenchyma and is not the next best imaging step.
- **Option B:** **Sputum Analysis** can be helpful in diagnosing infections but does not provide a direct assessment of the lung infiltrates seen on the chest X-ray.
- **Option D:** **Ultrasound of the Chest** might be useful in certain specific scenarios, such as detecting pleural effusions, but it is not as comprehensive as HRCT in evaluating lung infiltrates.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that HRCT of the chest is particularly valuable in patients with suspected interstitial lung disease or when the chest X-ray shows bilateral lung infiltrates of unclear etiology. It helps in early diagnosis and guiding treatment.

## **Correct Answer:** .