## **Core Concept**
The patient's presentation of breathlessness and chest pain after a period of prolonged bed rest raises suspicion for pulmonary embolism (PE), a condition where one or more pulmonary arteries are obstructed by a blood clot. This condition is a well-known complication of prolonged immobilization.

## **Why the Correct Answer is Right**
The correct answer, **D. CT Pulmonary Angiography (CTPA)**, is the most appropriate next investigation. Given the patient's symptoms and history of recent prolonged bed rest, a high clinical suspicion for PE exists. CTPA is the current recommended imaging test for diagnosing acute PE due to its high sensitivity and specificity, rapid acquisition, and wide availability. It directly visualizes the pulmonary arteries and any clots within them.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **Ventilation-Perfusion (V/Q) scan** can be used to diagnose PE, especially in patients with normal renal function and no contrast allergy, it is generally considered less specific and sensitive than CTPA and is usually reserved for cases where CTPA is contraindicated.
- **Option B:** An **Echocardiogram** can show indirect signs of PE, such as right ventricular strain, but it does not directly visualize the clot in the pulmonary arteries. It's useful in hemodynamically unstable patients but not the best initial test for diagnosing PE.
- **Option C:** A **D-dimer test** is a screening tool used to rule out PE in patients with low clinical probability. However, with a high clinical suspicion as in this case, a negative D-dimer does not rule out PE, and further imaging is required.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is the use of the **Wells' score** and **Revised Geneva score** to stratify patients' risk for PE before choosing imaging. For patients with high clinical probability, direct imaging with CTPA is often preferred over D-dimer testing.

## **Correct Answer:** **D. CT Pulmonary Angiography (CTPA)**