## **Core Concept**
Pulmonary thromboembolism (PTE) is a medical emergency characterized by the obstruction of the pulmonary arteries or its branches, usually by a venous thrombus. The diagnosis of PTE requires a combination of clinical assessment and imaging studies. The most commonly used investigations for suspected PTE include computed tomography pulmonary angiography (CTPA), ventilation-perfusion scan (V/Q scan), and others.

## **Why the Correct Answer is Right**
The correct answer, **D. CT Pulmonary Angiography**, is the most appropriate investigation for acute pulmonary thromboembolism. CT Pulmonary Angiography (CTPA) has become the first-line imaging test for diagnosing PTE due to its high sensitivity and specificity, rapid acquisition, and wide availability. It directly visualizes the pulmonary arteries and any thrombi within them, making it highly effective for diagnosing PTE.

## **Why Each Wrong Option is Incorrect**
- **Option A: Chest X-ray** is not specific for diagnosing PTE but may show nonspecific findings such as pleural effusion, atelectasis, or cardiomegaly. It is often used to rule out other causes of symptoms.
- **Option B: ECG** can show nonspecific changes in patients with PTE, such as sinus tachycardia, right ventricular strain, or T-wave inversion, but it is not diagnostic on its own.
- **Option C: Arterial Blood Gas (ABG)** may show hypoxemia and sometimes respiratory alkalosis in patients with PTE but is not a definitive diagnostic tool.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **Wells' score** and **Geneva score** are clinical prediction rules used to estimate the probability of PTE before imaging. However, imaging tests like CTPA are crucial for definitive diagnosis. Remember, CTPA is the gold standard for diagnosing acute PTE.

## **Correct Answer: D. CT Pulmonary Angiography**