**Core Concept**
Pulmonary thromboembolism (PTE) is a life-threatening condition where a blood clot obstructs blood flow in the pulmonary arteries. The diagnosis of PTE requires a combination of clinical assessment, imaging, and laboratory tests. In patients with high clinical suspicion of PTE, the best investigation is one that can quickly and accurately diagnose the condition, preventing further complications.

**Why the Correct Answer is Right**
The correct investigation for PTE is a computed tomography pulmonary angiography (CTPA). CTPA is a non-invasive imaging test that uses contrast dye to visualize the pulmonary arteries and detect clots. It is highly sensitive and specific for diagnosing PTE, especially in patients with high clinical suspicion. The test can also provide information on the location and size of the clot, which is essential for determining the severity of the condition and guiding treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not the best investigation for PTE as it may not show the clot directly and can be normal even in the presence of a significant clot.
**Option B:** ECG may show nonspecific changes in patients with PTE, such as right bundle branch block or T-wave inversion, but it is not diagnostic.
**Option C:** Ventilation-perfusion scan (V/Q scan) is an older imaging test that can be used to diagnose PTE, but it is less sensitive and specific than CTPA.

**Clinical Pearl / High-Yield Fact**
In patients with high clinical suspicion of PTE, a negative D-dimer test does not rule out the condition, especially in patients with recent surgery or trauma. D-dimer is a marker of fibrin degradation and can be elevated in various conditions, making it a non-specific test for PTE.

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