**Core Concept**
The muscles of the pollex (thumb) are primarily innervated by the median nerve, which is responsible for thumb abduction, opposition, and flexion. The median nerve is a branch of the brachial plexus and plays a crucial role in the motor and sensory functions of the hand.

**Why the Correct Answer is Right**
The adductor pollicis muscle is responsible for adduction of the thumb, which is the movement that brings the thumb towards the palmar surface of the hand. This muscle is unique in that it is innervated by the ulnar nerve, not the median nerve. The ulnar nerve supplies innervation to the adductor pollicis through its deep branch, which runs along the ulnar artery and vein. This distinction is important because it highlights the dual innervation of the hand, with the median nerve supplying most of the muscles involved in thumb movement, but the ulnar nerve supplying the adductor pollicis.

**Why Each Wrong Option is Incorrect**
**Option B:** Opponens pollicis is indeed supplied by the median nerve, which is responsible for the opposition movement of the thumb.
**Option C:** Abductor pollicis brevis is also supplied by the median nerve, which is responsible for the abduction movement of the thumb.
**Option D:** Flexor pollicis brevis is supplied by the median nerve, which is responsible for the flexion movement of the thumb.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the ulnar nerve supplies the adductor pollicis muscle, which is a key muscle involved in thumb adduction. This is an important distinction to make during clinical exams, as it can help to identify nerve damage or entrapment.

**✓ Correct Answer: A. Adductor pollicis**