Involuntary muscle found only in the heart.

• A. Skeletal muscle
• B. Aponeurosis
• C. Smooth muscle
• D. Cardiac muscle

Answer: (D)

Cardiac muscle is the involuntary muscle found only in the heart. It looks striated like skeletal muscle, but its contractions occur automatically thanks to the heart’s own conduction system, so you don’t control it consciously. Cardiac muscle cells are branched and tightly connected by intercalated discs that contain gap junctions and desmosomes, enabling ions to pass quickly between cells and producing a synchronized heartbeat. This unique arrangement lets the heart pump blood continuously.

The other options don’t fit: skeletal muscle is voluntary and attached to bones; smooth muscle is involuntary but lines hollow organs and vessels, not the heart; an aponeurosis is a flat sheet of connective tissue, not muscle tissue.