Which of the following is a primary function of the skin's sebaceous glands?

A. Saddle joint: Saddle joints allow biaxial movement (e.g., thumb carpometacarpal joint) but do not permit movement in all directions.
B. Pivot joint: Pivot joints allow rotation around a single axis (e.g., atlantoaxial joint) and therefore do not provide multiaxial movement.
C. Hinge joint: Hinge joints permit movement primarily in one plane (flexion/extension) like the elbow and do not allow movement in all directions.
D. Ball-and-socket joint: Ball-and-socket joints (for example the shoulder and hip) permit multiaxial movement including flexion/extension, abduction/adduction, and rotation, allowing the greatest directional range.

A. Radial groove: The radial (spiral) groove runs along the mid-shaft of the humerus and houses the radial nerve — injury here commonly causes wrist drop and sensory loss on the posterior forearm/hand.
B. Greater tubercle: The greater tubercle is on the proximal humerus and is the site for rotator cuff muscle attachments; it’s not the landmark for mid-shaft nerve injury.
C. Coracoid process: Part of the scapula (anterior shoulder), important for muscle/ligament attachment, not adjacent to the humeral mid-shaft or the radial nerve.
D. Medial epicondyle: A distal humeral landmark near the elbow where the ulnar nerve passes (cubital tunnel) — relevant to elbow/ulnar nerve injury, not mid-shaft radial nerve injury.