**Core Concept**
Cardiac tamponade is a life-threatening condition characterized by the accumulation of fluid in the pericardial space, leading to compression of the heart. This compression impairs cardiac filling and output, resulting in hypotension, jugular venous distension, and decreased cardiac output.

**Why the Correct Answer is Right**
Kussmaul's sign, which is an increase in jugular venous pressure (JVP) with inspiration, is typically seen in constrictive pericarditis, cardiac constriction, or right heart failure. In cardiac tamponade, the increased intrathoracic pressure with inspiration is transmitted to the pericardial space, causing further compression of the heart. However, the JVP may actually decrease with inspiration due to a decrease in intrathoracic pressure, which is known as pulsus paradoxus. This is a characteristic finding in cardiac tamponade. Kussmaul's sign is not typically seen in cardiac tamponade.

**Why Each Wrong Option is Incorrect**
**Option B:** Pulsus paradoxus is a characteristic finding in cardiac tamponade, where there is a greater than 10 mmHg decrease in systolic blood pressure with inspiration.
**Option C:** Electrical alternans is a sign of cardiac tamponade, where the QRS complex alternates in amplitude, due to the swinging motion of the heart within the pericardial fluid.
**Option D:** Right ventricular diastolic collapse on echocardiogram is a diagnostic finding in cardiac tamponade, indicating compression of the right ventricle during diastole.

**Clinical Pearl / High-Yield Fact**
In cardiac tamponade, the key clinical finding is pulsus paradoxus, which is a greater than 10 mmHg decrease in systolic blood pressure with inspiration. This occurs due to the increased intrathoracic pressure with inspiration, which further compresses the heart.

**Correct Answer:**
✓ Correct Answer: A. Kussmaul's sign