**Core Concept**
The median nerve, a branch of the brachial plexus, supplies the muscles of the anterior forearm and hand, controlling flexion and wrist movements. At the wrist, the median nerve is susceptible to injury, affecting motor function.

**Why the Correct Answer is Right**
The muscles primarily affected by median nerve injury at the wrist include the thenar muscles (thumb movements) and the lumbricals (finger flexion). The thenar muscles (thumb opposition, flexion, and abduction) are responsible for thumb movements, while lumbricals (II and III) assist in finger flexion. This nerve injury spares the flexor digitorum profundus muscle (III and IV) which controls finger flexion, as it receives its innervation from the ulnar nerve. The flexor pollicis longus, however, is innervated by the median nerve and may be affected.

**Why Each Wrong Option is Incorrect**
* **Option A:** Flexor digitorum profundus III and IV are spared as they receive innervation from the ulnar nerve, not the median nerve.
* **Option B:** Flexor pollicis longus is innervated by the median nerve and may be affected in median nerve injury.
* **Option C:** The lumbricals II and III are primarily responsible for finger flexion and are affected in median nerve injury.

**Clinical Pearl / High-Yield Fact**
When assessing median nerve injury, it's essential to remember that the flexor digitorum profundus (III and IV) is spared due to its ulnar nerve innervation, whereas the flexor pollicis longus and lumbricals II and III are affected.

**Correct Answer: A. Flexor digitorum profundus III and IV.**