The most common deformity of the spine is an abnormal lateral curvature called:

Choice A reason: While muscle names can reflect location (e.g., brachialis for the arm), the names "biceps," "triceps," and "quadriceps" specifically refer to the number of origins, not their anatomical position.

Choice B reason: "Biceps" means two heads, "triceps" means three heads, and "quadriceps" means four heads. These names are derived from the number of tendinous origins each muscle has, making this the correct classification.

Choice C reason: Muscle shape does influence naming in some cases (e.g., deltoid for triangular), but it is not the basis for naming biceps, triceps, or quadriceps.

Choice D reason: Fascicle arrangement refers to the pattern of muscle fibers (e.g., parallel, pennate), which affects function and strength but is not the naming basis for these specific muscles.

Choice A reason: The sarcomere is the smallest contractile unit of a skeletal muscle. It is composed of actin and myosin filaments arranged in a precise pattern that allows for muscle contraction. Sarcomeres are repeated along the length of a myofibril and are responsible for the striated appearance of skeletal muscle.

Choice B reason: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction. However, it is not the contractile unit itself.

Choice C reason: The sarcolemma is the plasma membrane of a muscle fiber. It plays a role in conducting electrical signals but is not the structural unit responsible for contraction.

Choice D reason: A myofibril is a long, cylindrical organelle found within muscle cells that contains many sarcomeres. While it contributes to muscle contraction, it is composed of multiple sarcomeres and is not the fundamental unit.