**Core Concept**
Deep vein thrombosis (DVT) and pulmonary embolism (PE) are serious complications that can occur in patients who have been immobile for an extended period. The clinical presentation of DVT and PE can be non-specific, and a normal chest X-ray does not rule out the diagnosis.

**Why the Correct Answer is Right**
The patient's symptoms of breathlessness and chest pain, combined with a history of immobility, make PE a high suspicion. The next step in investigation is to perform an ultrasound of the lower extremities to rule out DVT, which is a common precursor to PE. If DVT is present, it increases the likelihood of PE, and further imaging with a CT pulmonary angiogram (CTPA) may be warranted.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is already normal, so repeating it is not the next step.
**Option B:** Arterial blood gas (ABG) may show hypoxemia, but it does not diagnose DVT or PE.
**Option C:** ECG may show nonspecific changes, but it does not diagnose DVT or PE.

**Clinical Pearl / High-Yield Fact**
Always consider DVT and PE in patients with a history of immobility and non-specific symptoms of breathlessness and chest pain. A normal chest X-ray does not rule out PE, and further investigation with ultrasound and/or CTPA may be necessary.

**Correct Answer:** D.