With a client's arm at their side in a 90-degree angle and the palm facing up, what is the movement of the forearm that rotates the palm to face downward?

Pronation is the correct answer because it specifically refers to the movement that involves rotating the forearm so that the palm faces downward. When a person has their arm at a 90-degree angle with the palm facing upward (supinated position), any movement that turns the palm to face downward constitutes pronation. This action involves the radius and ulna bones of the forearm crossing over each other.

In this context, circumduction would not apply, as it involves a circular movement that combines flexion, extension, abduction, and adduction, typically of a shoulder joint or similar. Supination is the opposite of pronation and refers to the movement that turns the palm upward, which clarifies why it does not fit the described motion. Downward rotation typically refers to a different movement involving the scapula and does not pertain to the rotation of the forearm. Thus, the clear definition and mechanics of pronation make it the appropriate choice for this question.