## **Core Concept**
The patient presents with sudden onset of breathlessness and chest pain after a period of prolonged bed rest, which raises suspicion for pulmonary embolism (PE). The normal chest x-ray does not rule out PE. Given the clinical presentation, further diagnostic testing is crucial.

## **Why the Correct Answer is Right**
The most appropriate next step in investigation for suspected pulmonary embolism, especially when the chest x-ray is normal and clinical suspicion remains high, is to perform a **D-dimer test**. The D-dimer test is a useful screening tool for PE because it can help rule out the condition if the result is negative. A negative D-dimer result, in the context of low or moderate pre-test probability, can safely exclude PE. This test is particularly relevant in this scenario as the patient has risk factors for PE (prolonged bed rest), making **Option D** the correct next step.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a CT pulmonary angiogram (CTPA) is a definitive diagnostic test for PE, it is not typically the next step after a normal chest x-ray due to its cost, availability, and radiation exposure. It's usually reserved for when there's a high suspicion of PE or after initial screening tests.
- **Option B:** An electrocardiogram (ECG) can show nonspecific changes in PE but is not diagnostic on its own. It may be performed as part of the initial assessment but does not specifically investigate for PE.
- **Option C:** A blood gas analysis can provide information on oxygenation status and acid-base balance but is not specific for diagnosing PE.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the **Wells' score**, a clinical prediction rule for estimating the probability of PE. It includes criteria such as symptoms of DVT, immobilization or surgery in the previous four weeks, previous DVT/PE, hemoptysis, malignancy, and PE as likely or more likely than another diagnosis. For patients with a low or moderate Wells' score, a negative D-dimer can effectively rule out PE.

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