## **Core Concept**
The core concept here revolves around the diagnostic approach for a patient with suspected bronchogenic carcinoma (lung cancer). Given that tuberculosis (TB) has been ruled out, the focus shifts to investigations that can help confirm or rule out lung cancer, assess its extent, and guide management.

## **Why the Correct Answer is Right**
The correct answer, **.**, implies that the investigation to be done is likely a **CT (Computed Tomography) scan of the chest and abdomen**. A CT scan is crucial in the staging of lung cancer, providing detailed information about the tumor's size, location, and potential spread to lymph nodes or distant organs. This information is vital for determining the stage of cancer and planning appropriate treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a chest X-ray is often the first imaging done for respiratory symptoms, it lacks the sensitivity and specificity of a CT scan for detailed evaluation of lung nodules or masses and assessment of mediastinal lymphadenopathy.
- **Option B:** A PET (Positron Emission Tomography) scan can be useful in lung cancer staging, especially for assessing distant metastases and evaluating the tumor's metabolic activity. However, it is typically used in conjunction with a CT scan (PET-CT) rather than as a standalone initial investigation.
- **Option C:** An MRI (Magnetic Resonance Imaging) of the brain might be considered for evaluating brain metastases but is not the initial investigation of choice for suspected bronchogenic carcinoma.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for suspected lung cancer, a **contrast-enhanced CT scan of the chest, abdomen, and pelvis** is often recommended as part of the initial staging workup. This helps in assessing the primary tumor, regional lymph nodes, and distant metastases.

## **Correct Answer:** . CT chest and abdomen.