**Core Concept**
The patient's presentation suggests a condition affecting the heart, lungs, and possibly the pericardium, given the symptoms of dyspnea, pedal edema, elevated JVP, and diminished heart sounds. **Cardiac tamponade** or **constrictive pericarditis** could be considered, but the key is the negative Kussmaul's sign, which helps differentiate these conditions.

**Why the Correct Answer is Right**
A negative Kussmaul's sign (decrease in JVP during inspiration) is typically seen in **cardiac tamponade**, where fluid accumulation in the pericardial space compresses the heart, impeding its filling. The variability in QRS complex height on the ECG can be due to **electrical alternans**, a phenomenon sometimes observed in cardiac tamponade due to the heart's swinging motion within the fluid-filled pericardium.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it suggested constrictive pericarditis, as this condition typically presents with a positive Kussmaul's sign (increase in JVP during inspiration).
**Option B:** Might be incorrect if it implied a primary pulmonary issue without cardiac involvement, as the patient's symptoms and signs suggest cardiac compromise.
**Option C:** Could be incorrect if it pointed towards a condition not explaining the full spectrum of symptoms, such as isolated heart failure without considering the pericardial involvement.
**Option D:** Would be incorrect if it suggested a diagnosis not supported by the combination of clinical findings and ECG changes.

**Clinical Pearl / High-Yield Fact**
The presence of a negative Kussmaul's sign in a patient with suspected pericardial disease points more towards **cardiac tamponade**. Remember, electrical alternans on an ECG can be a clue to this diagnosis.

**Correct Answer:** D. Cardiac Tamponade.