**Core Concept**
Deep vein thrombosis (DVT) is a common complication in immobile patients, leading to pulmonary embolism (PE) which can cause sudden onset breathlessness and chest pain. The clinical presentation of PE is often nonspecific, and diagnosis relies heavily on imaging studies.

**Why the Correct Answer is Right**
Given the clinical presentation of sudden onset breathlessness and chest pain in a bedridden patient, the next investigation should be a computed tomography pulmonary angiogram (CTPA) to rule out pulmonary embolism. CTPA is a highly sensitive and specific test for diagnosing PE. It allows for direct visualization of the pulmonary arteries and can identify thrombi. The normal chest X-ray does not rule out PE, as many patients with PE have a normal chest X-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** ECG is not the next investigation of choice for diagnosing PE. While an ECG may show signs of right ventricular strain or arrhythmias in the setting of PE, it is not specific or sensitive enough to diagnose PE.

**Option B:** Blood tests, such as D-dimer, can be used to support the diagnosis of PE but are not the next investigation of choice. D-dimer is a nonspecific marker of thrombosis and can be elevated in a wide range of conditions.

**Option C:** Chest X-ray was already performed and was normal. A repeat chest X-ray would not provide additional information.

**Clinical Pearl / High-Yield Fact**
In patients with suspected PE, a normal chest X-ray does not rule out the diagnosis, and a high index of suspicion should lead to further investigation, such as a CTPA.

**Correct Answer: CTPA**