**Core Concept:**
The question pertains to the investigation of choice for suspected pulmonary embolism (PE), a potentially life-threatening condition characterized by blockage of a pulmonary artery by a blood clot. PE can cause serious complications including respiratory distress, hypoxia, and heart failure. Diagnosis requires a combination of clinical suspicion and appropriate imaging techniques.

**Why the Correct Answer is Right:**
The correct answer, CT pulmonary angiography (CTPA), is the most sensitive and specific imaging modality for diagnosing PE. It uses computed tomography (CT) to visualize the pulmonary vasculature, allowing for detection of the clot and exclusion of other causes of pulmonary opacities. CTPA is preferred over other options because:

1. **Option A (Venous Duplex Ultrasound):** This technique is primarily used to evaluate deep vein thrombosis (DVT), the precursor to PE. While it may identify DVT, it lacks the sensitivity and specificity for PE diagnosis.
2. **Option B (Echocardiography):** Echocardiography focuses on assessing the heart's function and structure. It may reveal right ventricular dysfunction due to PE, but it does not directly visualize the pulmonary vasculature for clot detection.
3. **Option C (Plain Chest X-ray):** Plain chest X-ray is a basic imaging modality with limited sensitivity and specificity for PE diagnosis. It may show pleural effusion, pneumothorax, or pulmonary edema but is inadequate for detecting PE.

**Why Each Wrong Option is Incorrect:**
1. **Option A (Venous Duplex Ultrasound):** Although venous duplex ultrasound is useful for assessing deep vein thrombosis (DVT), it lacks sensitivity and specificity for pulmonary embolism (PE) diagnosis. The absence of a test does not definitively rule out PE, as it is not a direct imaging technique for the pulmonary vasculature.
2. **Option B (Echocardiography):** Echocardiography focuses on assessing the heart's function and structure. It may reveal right ventricular dysfunction due to PE, but it does not directly visualize the pulmonary vasculature for clot detection.
3. **Option C (Plain Chest X-ray):** Plain chest X-ray has limited sensitivity and specificity for pulmonary embolism (PE) diagnosis. It may show pleural effusion, pneumothorax, or pulmonary edema but is inadequate for detecting PE.

**Clinical Pearls and High-Yield Facts:**
Diagnosing pulmonary embolism involves a thorough clinical evaluation, followed by appropriate imaging tests. In case of inconclusive imaging findings, further diagnostic workup should be pursued, including D-dimer testing, ventilation-perfusion scan (V/Q scan), or CT pulmonary angiography.

In conclusion, CT pulmonary angiography (CTPA) is the most accurate imaging test for pulmonary embolism, allowing direct visualization of the pulmonary vasculature and detection of clot presence. It provides a definitive diagnosis and guides management decisions, such as thrombolysis or anticoagulation therapy.