**Core Concept**
Deep vein thrombosis (DVT) is a common complication in immobile patients, particularly those on bed rest for an extended period. The risk of DVT increases with factors such as age, immobility, and malignancy.

**Why the Correct Answer is Right**
In this scenario, the patient's symptoms of breathlessness and chest pain, combined with a history of prolonged bed rest, suggest a possible pulmonary embolism (PE). However, the normal chest X-ray does not rule out PE. The next investigation should be a D-dimer test to assess for the presence of fibrin degradation products, which is a sensitive but non-specific marker for thrombosis. A positive D-dimer would necessitate further imaging with a CT pulmonary angiogram (CTPA) to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** An electrocardiogram (ECG) may show nonspecific changes in patients with PE, but it is not the next investigation in this scenario.
**Option B:** A complete blood count (CBC) may show leukocytosis in response to stress or infection, but it is not specific for diagnosing DVT or PE.
**Option C:** A ventilation-perfusion scan may be useful in patients with a contraindication to CTPA, but it is not the next step in this scenario.

**Clinical Pearl / High-Yield Fact**
In patients with suspected PE, a negative D-dimer result can rule out PE with a high degree of certainty, but a positive result only warrants further investigation.

**Correct Answer:** D. D-dimer test.