Which statement best describes a diarthrotic joint?

• A. They are typically immovable.
• B. They are bound by cartilage with no joint cavity.
• C. They contain a synovial cavity and are usually freely movable.
• D. They occur only in the skull.

Answer: (C)

Diarthrotic joints are synovial joints that have a fluid-filled synovial cavity, which allows a wide range of movement. The ends of the bones are covered with articular cartilage and enclosed by an articular capsule lined with a synovial membrane that secretes lubricating synovial fluid. This cavity and lubrication enable the joints to move freely and smoothly, supporting diverse movements like bending, extending, rotating, and gliding.

This is why the statement is best: it emphasizes the presence of a synovial cavity and the typical freedom of movement. In contrast, joints bound by cartilage with no cavity are cartilaginous joints, which are usually immobile or only slightly movable, not diarthrotic. And the idea that diarthrotic joints occur only in the skull isn’t correct, since these highly mobile joints are common in the limbs and trunk (for example, elbow, knee, shoulder, hip).