## **Core Concept**
Froment's sign is a clinical test used to assess the integrity of a specific nerve. It involves the patient's ability to adduct the thumb, which is primarily controlled by the **adductor pollicis muscle**. This muscle is innervated by the **ulnar nerve**.

## **Why the Correct Answer is Right**
The correct answer, **ulnar nerve**, is right because Froment's sign specifically tests for the function of the ulnar nerve. When the ulnar nerve is damaged, the adductor pollicis muscle is weakened or paralyzed, leading to a compensatory action where the flexor pollicis longus (innervated by the median nerve) is used to try to adduct the thumb. This results in a positive Froment's sign, where the patient is unable to properly adduct the thumb and instead uses the flexor pollicis longus, causing the interphalangeal joint of the thumb to flex during attempted adduction.

## **Why Each Wrong Option is Incorrect**
- **Option A: Median nerve** - This is incorrect because the median nerve primarily controls muscles involved in thumb opposition (opponens pollicis) and flexion (flexor pollicis brevis), not adduction.
- **Option B: Radial nerve** - This is incorrect because the radial nerve primarily controls wrist and finger extension (extensor muscles of the wrist and fingers), not thumb adduction.
- **Option D: Anterior interosseous nerve** - This is incorrect because the anterior interosseous nerve, a branch of the median nerve, primarily controls deep finger flexors (flexor pollicis longus, lateral half of flexor digitorum profundus) and pronator quadratus, not thumb adduction.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that a positive Froment's sign indicates **ulnar nerve palsy**. This can be remembered by associating the inability to adduct the thumb with ulnar nerve dysfunction. Clinically, this is significant because ulnar nerve palsy can lead to significant hand dysfunction, including weakness in grip strength and difficulty with fine motor tasks.

## **Correct Answer: C. Ulnar nerve**