Formed in muscle fibers by myofilaments of actin and myosin arranged in an organized manner; composed of hundreds of sarcomeres end-to-end

• A. Myofibril
• B. Sarcomere
• C. Tendon
• D. Fascicle

Answer: (A)

Inside a muscle fiber, the contractile engine is a myofibril. Myofibrils are long, rod-like structures made up of myofilaments—actin and myosin—organized into repeating units called sarcomeres. When hundreds of these sarcomeres align end-to-end along a single myofibril, they form a long chain that runs the length of the fiber, giving the fiber its organized, contractile structure. This arrangement lets the muscle shorten as the sarcomeres contract, with the actin and myosin filaments sliding past one another. In contrast, a tendon is connective tissue that attaches muscle to bone, and a fascicle is a bundle of muscle fibers. So the description provided points to a myofibril.