**Core Concept**
Froment's sign is a clinical test used to assess the integrity of the ulnar nerve, specifically the adductor pollicis muscle, which is innervated by the ulnar nerve. It involves assessing the patient's ability to hold a piece of paper between their thumb and index finger.

**Why the Correct Answer is Right**
When performing Froment's sign, the patient is asked to hold a piece of paper between their thumb and index finger while the examiner tries to pull the paper away. In a normal individual, the adductor pollicis muscle contracts to maintain grip. However, if the ulnar nerve is damaged, the patient may use their flexor pollicis longus muscle (innervated by the median nerve) to compensate for the weakness in adductor pollicis, leading to an abnormal Froment's sign. This is because the flexor pollicis longus muscle is not typically involved in thumb adduction, but can be recruited in cases of ulnar nerve injury.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Froment's sign is specifically related to the ulnar nerve and not to the median nerve, which is responsible for the flexor pollicis longus muscle.
**Option B:** This option is incorrect because Froment's sign is not a test of the radial nerve, which is responsible for wrist and finger extension, not thumb adduction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Froment's sign can be a sensitive indicator of ulnar nerve damage, but it's not specific, and other nerve injuries or conditions can also cause an abnormal result. Always consider the clinical context and other examination findings.

**Correct Answer:** D.