## Core Concept
The diagnosis of pulmonary embolism (PE) involves various imaging techniques, each with its own strengths and limitations. The gold standard investigation is critical for confirming the presence of PE, especially in high-risk patients. Computed Tomography Pulmonary Angiography (CTPA) and ventilation-perfusion scan (V/Q scan) are commonly used, but the most definitive method involves direct visualization of the pulmonary arteries.

## Why the Correct Answer is Right
The correct answer, **CT Pulmonary Angiography (CTPA)**, is considered the gold standard for diagnosing pulmonary embolism. CTPA directly visualizes the pulmonary arteries and any emboli within them. It provides detailed images that can show the location and size of the clot, which is crucial for determining the severity of the PE and guiding treatment. CTPA is widely available, quick, and can be performed in emergency settings, making it highly valuable for acute diagnosis.

## Why Each Wrong Option is Incorrect
- **Option A: Chest X-ray** is not specific for PE and is often used to rule out other causes of symptoms that might mimic PE, such as pneumonia or pneumothorax. While it can show indirect signs suggestive of PE, such as the Westermark sign or Hampton's hump, these are not definitive.
- **Option B: Ventilation-Perfusion Scan (V/Q Scan)** is used, especially in patients where CTPA is contraindicated, such as severe renal impairment or pregnancy. However, it is not as specific as CTPA and can yield indeterminate results.
- **Option D: D-dimer Test** is a screening tool used to rule out PE in low-risk patients. A negative result can help exclude PE, but a positive result is non-specific and requires further imaging for confirmation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while CTPA is the gold standard, it's essential to assess the patient's renal function before administering contrast to minimize the risk of contrast-induced nephropathy. Also, in patients with a high clinical suspicion of PE and a negative CTPA, further evaluation with other modalities like MRI or pulmonary angiography might be considered, though the latter is invasive.

## Correct Answer: C. CT Pulmonary Angiography (CTPA).