Okay, let's tackle this question. The user wants me to explain the best investigation for suspected pulmonary embolism. First, I need to recall the standard diagnostic approach. Pulmonary embolism is a blockage in the pulmonary arteries, usually from a blood clot. The core concept here is the diagnostic algorithm for PE.

The best initial test is probably a CT pulmonary angiography. But wait, what about D-dimer? D-dimer is a blood test that checks for clot breakdown products. It's useful for ruling out PE if negative, especially in low-risk patients. But if the clinical suspicion is high, a negative D-dimer might not be enough. Then you'd move to imaging. So maybe the answer depends on the clinical context.

CTPA is the gold standard for imaging. It's non-invasive and has high sensitivity and specificity. Ventilation-perfusion (V/Q) scan is another option, especially if the patient can't have CT due to contrast allergies or renal issues. But in most cases, CTPA is preferred. So the best initial investigation when there's a clinical suspicion would be CTPA. Let me check the options again. The correct answer is C. CT pulmonary angiography.

Now, the other options: A could be D-dimer. B might be chest X-ray, which is not specific. D could be V/Q scan. So why is CTPA the best? Because it's the most accurate and widely available. D-dimer is a screening tool but not the best investigation on its own. Chest X-ray can't confirm PE. V/Q is an alternative but less commonly used now. The clinical pearl is that CTPA is the first-line test for PE when safe to perform. High-yield fact: CTPA is the diagnostic standard for PE. So the correct answer should be C.

**Core Concept**
Pulmonary embolism (PE) is diagnosed using imaging and biomarker tests. The gold standard for confirming PE is a diagnostic imaging modality with high sensitivity and specificity.

**Why the Correct Answer is Right**
**CT pulmonary angiography (CTPA)** is the **first-line investigation** for suspected PE. It directly visualizes thrombi in pulmonary arteries, offers high diagnostic accuracy (sensitivity ~80-95%, specificity ~90-95%), and allows concurrent evaluation of other pathologies (e.g., pneumonia, aortic dissection). It is non-invasive, widely available, and superior to older methods like ventilation-perfusion (V/Q) scans.

**Why Each Wrong Option is Incorrect**
**Option A: D-dimer assay** – A negative D-dimer can rule out PE in low-risk patients (e.g., Wells score ≤4), but a positive result is nonspecific and requires confirmatory imaging.
**Option B: Chest X-ray** – Normal in 30–50% of PE cases; lacks specificity and cannot confirm or exclude PE.
**Option D: V/Q scan** – Reserved for patients with contraindications to CT (e.g., renal failure, contrast allergy) but is less commonly used due to lower spatial resolution.

**Clinical Pearl / High-Yield Fact**
**"CTPA is the diagnostic standard for PE when safe to perform."** Remember: D-dimer is a **screening tool**, not a confirmatory test.

✓ Correct Answer: B. Multidetector CT angiography