**Core Concept**
The patient's presentation with sudden onset of breathlessness, haemoptysis, and chest pain, in the context of nephrotic syndrome and recent long-distance air travel, suggests a pulmonary embolism (PE). This condition is characterized by the obstruction of a pulmonary artery by a blood clot, which can lead to hypoxemia, shock, and potentially fatal outcomes.

**Why the Correct Answer is Right**
Pulmonary embolism is a common cause of sudden death in patients with nephrotic syndrome, due to the increased risk of venous thromboembolism associated with immobility and hypercoagulability. The patient's recent air travel may have exacerbated this risk, leading to a significant clot formation in the pulmonary arteries. The ECG showing sinus tachycardia is a non-specific finding but consistent with the body's response to shock and hypoxia. The normal chest X-ray does not rule out PE, as it may not be visible until significant clot burden is present.

**Why Each Wrong Option is Incorrect**
**Option A:** Pulmonary edema is unlikely in this context, as the patient's chest X-ray is normal and there is no mention of left ventricular dysfunction or fluid overload.
**Option B:** Acute myocardial infarction is less likely, given the normal ECG and lack of chest pain pattern consistent with ischemia.
**Option C:** Pneumonia is not supported by the normal chest X-ray and absence of fever or respiratory symptoms.
**Option D:** Cardiac tamponade is a possibility in patients with nephrotic syndrome, but the normal ECG and lack of specific symptoms make it less likely.

**Clinical Pearl / High-Yield Fact**
In patients with nephrotic syndrome, immobility and hypercoagulability increase the risk of venous thromboembolism, making PE a common and potentially fatal complication.

**Correct Answer:** D.