## Core Concept
Froment's sign is a clinical test used to assess the integrity of a specific nerve that controls thumb adduction. The test involves asking the patient to grasp a piece of paper between their thumb and index finger while the examiner tries to pull it away. The patient is then asked to release the paper. Normally, the adductor pollicis muscle, innervated by the **ulnar nerve**, would facilitate this action.

## Why the Correct Answer is Right
The correct answer, **ulnar nerve**, is right because Froment's sign is positive (indicating a problem) when there is weakness in the adductor pollicis muscle, which is innervated by the ulnar nerve. A positive Froment's sign suggests ulnar nerve injury. During the test, if the adductor pollicis is weak, the patient compensates by using the flexor pollicis longus (innervated by the median nerve), resulting in flexion of the interphalangeal joint of the thumb as they try to grasp the paper.

## Why Each Wrong Option is Incorrect
- **Option A:** Incorrect because the median nerve primarily controls muscles of the anterior forearm and thenar eminence, not thumb adduction.
- **Option B:** Incorrect as there seems to be a typographical error here, but if referencing another nerve, it does not accurately relate to thumb adduction.
- **Option D:** Incorrect because the radial nerve primarily controls wrist and finger extension and does not innervate the adductor pollicis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that a positive Froment's sign is indicative of **ulnar nerve damage**. This is crucial for diagnosing ulnar nerve injuries or neuropathies, which can result from various causes including trauma, compression, or systemic diseases.

## Correct Answer Line
**Correct Answer: C. ulnar nerve**