**Core Concept**
Pulmonary embolism (PE) is a life-threatening condition characterized by the obstruction of a pulmonary artery by a blood clot. Accurate diagnosis is crucial to prevent further complications. The investigation of choice for PE involves a combination of clinical assessment, imaging techniques, and biomarkers.

**Why the Correct Answer is Right**
The investigation of choice for pulmonary embolism is the computed tomography pulmonary angiography (CTPA). CTPA is a non-invasive imaging technique that uses contrast dye to visualize the pulmonary arteries and detect clots. It has high sensitivity and specificity for diagnosing PE, especially in patients with a high clinical probability. The CT scan also allows for the assessment of other conditions such as pulmonary infarction, atelectasis, and pleural effusion.

**Why Each Wrong Option is Incorrect**
**Option A:** Pulmonary angiography is an invasive procedure that involves injecting a contrast dye directly into the pulmonary artery. It is generally reserved for patients who have a high clinical probability of PE and require further evaluation.
**Option B:** Ventilation-perfusion (V/Q) scan is a nuclear medicine test that assesses the ventilation and perfusion of the lungs. While it can be used to diagnose PE, especially in patients who are allergic to iodine or have renal impairment, it is less sensitive than CTPA.
**Option C:** D-dimer is a biomarker that is elevated in patients with PE. However, it is not specific for PE and can be elevated in other conditions such as pregnancy, trauma, and malignancy.

**Clinical Pearl / High-Yield Fact**
Remember that the Wells' score is a clinical prediction rule that helps to estimate the probability of PE in patients. A score of 4 or higher indicates a high clinical probability of PE, which warrants further investigation with CTPA.

**Correct Answer:** C. Computed tomography pulmonary angiography (CTPA)