The anterior interosseous nerve innervates how many muscles?

The anterior interosseous nerve, a branch of the median nerve, is primarily responsible for innervating certain muscles in the forearm that are crucial for flexion. The correct understanding is that it innervates 2.5 muscles: the flexor pollicis longus, the lateral half of the flexor digitorum profundus, and the pronator quadratus.

The reason it is referred to as "2.5 muscles" is because the flexor digitorum profundus has two parts, with the anterior interosseous nerve only innervating the lateral half (which flexes the index and middle fingers), while the medial half is innervated by the ulnar nerve. The other two muscles, flexor pollicis longus and pronator quadratus, are fully innervated by the anterior interosseous nerve.

In the provided options, "2.5 muscles" accurately reflects this division, highlighting the intricate nature of nerve innervation in this region. The other options, which suggest whole numbers or incorrect fractions, do not account for the context of the flexor digitorum profundus and therefore do not represent the anatomical reality of the anterior interosseous nerve's innervation.