**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 and impairment of cardiac function. This results in reduced cardiac output, decreased blood pressure, and ultimately shock.

**Why the Correct Answer is Right**
Cardiac tamponade is diagnosed based on the presence of Beck's triad, which includes hypotension, muffled heart sounds, and jugular venous distension. These symptoms occur due to the pressure exerted by the fluid in the pericardial space on the heart, preventing it from expanding and filling properly. The presence of Beck's triad is a hallmark of cardiac tamponade, making it a critical aspect of diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Pulsus paradoxus is often seen in cardiac tamponade, as the increased pressure in the pericardial space causes a significant drop in systemic blood pressure during inspiration. This is due to the compression of the heart, which leads to a decrease in cardiac output during inspiration.

**Option B:** Electrical alternans can also be observed in cardiac tamponade, as the heart moves back and forth within the fluid-filled pericardial space, causing the electrical activity to alternate between the two sides of the heart.

**Option C:** Pericardial effusion is a key feature of cardiac tamponade, as the fluid accumulation in the pericardial space compresses the heart and impairs its function.

**Clinical Pearl / High-Yield Fact**
Beck's triad is a classic clinical finding in cardiac tamponade, and its presence should prompt immediate evaluation and treatment to prevent cardiac arrest and death.

**Correct Answer: D.**