**Core Concept**
The patient's symptoms of breathlessness and chest pain, despite a normal chest X-ray, suggest a pulmonary embolism (PE) or other cardiopulmonary conditions that do not always show radiographic abnormalities. The clinical presentation warrants further diagnostic workup.

**Why the Correct Answer is Right**
In this scenario, the patient's history of prolonged bed rest increases the risk of venous thromboembolism (VTE), including pulmonary embolism. The normal chest X-ray does not rule out PE, as imaging may not be sensitive enough to detect smaller clots. A d-dimer test is a reasonable next step to rule out PE, especially in a patient with high clinical suspicion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a CT angiogram is a more specific test for diagnosing PE, but it is not the initial step in evaluation. It is typically used after initial screening tests like the d-dimer.

**Option B:** This option is incorrect because an ECG can show signs of ischemia or arrhythmias but does not directly diagnose PE. While an ECG is essential in the initial assessment, it is not the next step after a normal chest X-ray.

**Option C:** This option is incorrect because a chest X-ray is already normal, making it unlikely to provide new information. Repeating the chest X-ray is not a productive next step in this case.

**Option D:** This option is incorrect because a ventilation-perfusion scan (V/Q scan) is not typically used as an initial screening test for PE. It may be used in patients with contraindications to CT angiography or when CT is not available.

**Clinical Pearl / High-Yield Fact**
In patients with suspected PE and a normal chest X-ray, a d-dimer test can be a useful initial screening tool to rule out VTE. However, it is essential to consider the patient's clinical context, including risk factors and symptoms, when interpreting the results.

**Correct Answer: B. ECG.**