Final answer:
A dislocated radial head primarily affects the ability to perform pronation and supination of the forearm.
Step-by-step explanation:
A patient with a dislocated radial head would have the most difficulty performing pronation and supination movements of the forearm. Pronation moves the forearm from the supinated (palm forward) position to the pronated (palm backward) position, involving rotation of the radius at the proximal radioulnar joint and movement of the radius at the distal radioulnar joint. Supination is the opposite motion, where rotation of the radius brings the forearm back to the supinated position.