Which description matches the muscular system?

• A. A system that produces contractions that result in movements, maintaining posture, producing changes in shape, and moving fluids through hollow tissues or organs
• B. Striated fibers attached to bone and responsible for movement of the skeleton
• C. Striated fibers that make up the walls of the heart
• D. Unstriated fibers lining various organs and lining blood vessels

Answer: (C)

The key idea is that the muscular system includes cardiac muscle, which forms the walls of the heart. Cardiac muscle is striated like skeletal muscle, but it functions involuntarily to produce the heart’s rhythmic contractions that pump blood. This description matches cardiac muscle—the tissue that makes up the heart walls (the myocardium). It’s distinct from skeletal muscle, which is striated fibers attached to bone to move the skeleton, and from smooth muscle, which is unstriated and lines hollow organs and blood vessels.