A 25-year-old male comes to his physician for a follow up visit with weakness of his right hand. Two months ago, he suffered a motor vehicle accident in which his right hand was affected. On examination, he is unable to flex the metacarpophalangeal joints of the ring and index fingers, and is unable to adduct the same fingers. Which of the following muscles are most likely paralyzed?

**Core Concept**
The question requires the identification of the muscles responsible for the specific motor functions of the hand, including flexion of the metacarpophalangeal joints and adduction of the fingers. This involves understanding the anatomy and function of the thenar and hypothenar muscles.

**Why the Correct Answer is Right**
The muscles responsible for flexion of the metacarpophalangeal joints of the ring and index fingers are the lumbricals to these fingers. However, the primary muscles responsible for adduction of these fingers are the adductor pollicis and the third and fourth lumbricals. The inability to perform these actions suggests a lesion affecting the ulnar nerve, which innervates these muscles. The ulnar nerve also innervates the interosseous muscles, which are involved in finger adduction. A lesion of the ulnar nerve would result in weakness or paralysis of these muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** The flexor digitorum profundus muscle is responsible for flexion of the distal interphalangeal joints, not the metacarpophalangeal joints.
**Option B:** The flexor pollicis longus muscle is responsible for flexion of the thumb, not the index or ring fingers.
**Option C:** The abductor pollicis longus muscle is responsible for abduction of the thumb, not adduction of the fingers.

**Clinical Pearl / High-Yield Fact**
In clinical practice, a lesion of the ulnar nerve should be suspected in patients with weakness or paralysis of the intrinsic muscles of the hand, including the interosseous muscles and the adductor pollicis.

**Correct Answer: D. The ulnar nerve innervates the adductor pollicis and the third and fourth lumbricals.**

✓ Correct Answer: B. Palmar interossei