**Question:** A patient presents with a solitary pulmonary nodule (SPN) on x-ray. The best investigation to come to a diagnosis would be:

A. Chest CT scan
B. Chest X-ray
C. Lung biopsy
D. Chest USG

**Correct Answer:** A. Chest CT scan

**Core Concept:**
A solitary pulmonary nodule (SPN) is a radiological finding on chest imaging, such as chest X-ray or CT scan, representing a pulmonary lesion smaller than 3 cm in diameter. The choice of investigation depends on the characteristics of the nodule, including size, location, and appearance (solid, part-solid, or ground-glass opacity). Chest CT scan is a more sensitive and specific tool for detecting SPNs compared to chest X-ray, which is less detailed and limited by superimposition of تشريحية structures. Chest CT scan allows for better characterization of nodules and helps in ruling out other causes of pulmonary nodules, such as infections or pulmonary embolism, which might be confused with malignant lesions.

**Why the Correct Answer is Right:**
A chest CT scan is the best choice for diagnosing a solitary pulmonary nodule because it provides detailed information about the nodule's size, shape, and density, which can help differentiate it from other causes of pulmonary nodules. CT scan can reveal the presence of a halo sign, spiculation, or pleural retraction, which are characteristic features of malignancy. A chest X-ray is less sensitive and specific in detecting small nodules due to its limited detail and superimposition of anatomical structures. Chest USG is not a suitable choice as it is less accurate in detecting pulmonary lesions and is not routinely performed for pulmonary lesions.

**Why Each Wrong Option is Incorrect:**

**Option B (Chest X-ray):** Chest X-ray is less sensitive and specific in detecting small pulmonary nodules due to its limited detail and superimposition of anatomical structures. Chest X-ray is useful for ruling out certain causes like pulmonary embolism or infection but is not suitable for the primary evaluation of a solitary pulmonary nodule.

**Option D (Chest USG):** Chest USG is not a suitable choice as it is less accurate in detecting pulmonary lesions and is not routinely performed for pulmonary lesions. Chest USG focuses on evaluating pleural effusion, pneumothorax, or mediastinal lymph nodes, not pulmonary nodules.

**Option C (Lung biopsy):** Although lung biopsy is a definitive diagnostic tool, it is an invasive procedure with potential complications and should be reserved for cases where non-invasive imaging modalities fail to provide a definitive diagnosis or when a tissue diagnosis is necessary for treatment planning. Chest CT scan provides a non-invasive method for assessing pulmonary nodules, allowing for a more detailed evaluation of the nodule's characteristics (size, shape, calcification, and the possibility of lung cancer).

**Clinical Pearl:**

A solitary pulmonary nodule can be classified as benign or malignant based on its size, shape, and other imaging features. A nodule that is less than 5mm in diameter is more likely