**Core Concept**
The patient presents with signs and symptoms suggestive of right heart failure, which is often a complication of metastatic lung cancer. Right heart failure can be due to primary causes such as cardiomyopathy or secondary causes like pulmonary embolism, pulmonary hypertension, or external compression of the heart by a tumor.

**Why the Correct Answer is Right**
The patient's presentation of dyspnea, pedal edema, and elevated JVP with a negative Kussmaul sign is highly suggestive of right heart failure. The negative Kussmaul sign, which is an increase in JVP with inspiration, is often seen in constrictive pericarditis but in this case, it's more likely to be due to right ventricular dysfunction. The ECG findings of QRS complexes of variable height suggest right bundle branch block, which is consistent with right ventricular strain or failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** While pulmonary embolism is a possible cause of right heart failure, the negative Kussmaul sign and the ECG findings make it less likely in this case.
* **Option B:** Cardiomyopathy can cause right heart failure, but the clinical context of metastatic lung cancer and the specific ECG findings make this less likely.
* **Option D:** While external compression of the heart by a tumor can cause right heart failure, the negative Kussmaul sign and the ECG findings do not specifically suggest this diagnosis.

**Clinical Pearl / High-Yield Fact**
In patients with metastatic lung cancer, right heart failure is a common complication, and it can be due to a combination of factors including pulmonary embolism, pulmonary hypertension, and external compression of the heart by a tumor. The clinical presentation may be subtle, and a high index of suspicion is required for early diagnosis and management.

**Correct Answer:** C. Constrictive pericarditis is incorrect because of the negative Kussmaul sign.