**Core Concept**
Pulmonary embolism (PE) diagnosis requires a non-invasive imaging modality that can quickly and accurately identify clots in the pulmonary arteries. The ideal investigation should have high sensitivity and specificity to avoid false positives and false negatives.

**Why the Correct Answer is Right**
Computed Tomography Pulmonary Angiography (CTPA) is the investigation of choice for diagnosing pulmonary embolism due to its high sensitivity (95-98%) and specificity (99-100%) for detecting clots in the pulmonary arteries. CTPA uses a combination of non-contrast and contrast-enhanced CT scans to visualize the pulmonary arteries and detect filling defects, which indicate a clot. This modality is particularly useful because it can also evaluate for other causes of acute pulmonary symptoms, such as pneumonia or pneumothorax.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not sensitive enough to detect pulmonary embolism, as it may show nonspecific findings such as an enlarged heart or pleural effusion, but cannot visualize the pulmonary arteries directly.

**Option B:** Ventilation-Perfusion (V/Q) scan is an alternative to CTPA, but it is less sensitive and more time-consuming, making it a secondary choice for patients who cannot undergo CT scanning.

**Option C:** Magnetic Resonance Angiography (MRA) is not typically used for pulmonary embolism diagnosis due to its lower sensitivity and specificity compared to CTPA.

**Clinical Pearl / High-Yield Fact**
Remember that CTPA is not only used to diagnose pulmonary embolism but also to evaluate for other causes of acute pulmonary symptoms. A thorough understanding of the imaging findings and correlation with clinical symptoms is essential for accurate diagnosis.

**Correct Answer: C. Computed Tomography Pulmonary Angiography (CTPA).**