**Core Concept**
Froment's test is a clinical examination tool used to assess the function of the ulnar nerve, specifically the adductor pollicis muscle, which is innervated by the ulnar nerve. This test evaluates the muscle's ability to adduct the thumb against resistance, providing insight into ulnar nerve integrity.

**Why the Correct Answer is Right**
During Froment's test, the patient is asked to hold a piece of paper between the thumb and index finger and then asked to release it. The examiner provides resistance to the thumb, and the patient is asked to adduct the thumb against the resistance. In a normal individual, the adductor pollicis muscle, innervated by the ulnar nerve, contracts and helps to adduct the thumb. If the ulnar nerve is injured, the patient will compensate by using the flexor pollicis longus muscle, innervated by the median nerve, to adduct the thumb, resulting in a positive Froment's sign.

**Why Each Wrong Option is Incorrect**
**Option B:** Median nerve injury would result in weakness or paralysis of the flexor pollicis longus muscle, but it would not directly affect the adductor pollicis muscle, which is responsible for the positive Froment's sign.
**Option C:** Radial nerve injury would affect wrist extension and finger extension, but it would not directly impact the adductor pollicis muscle or the clinical finding of Froment's test.
**Option D:** Axillary nerve injury would affect shoulder abduction and deltoid function, which is unrelated to the clinical finding of Froment's test.

**Clinical Pearl / High-Yield Fact**
A positive Froment's sign is a sensitive indicator of ulnar nerve injury, but it can be a false negative in cases of ulnar nerve palsy with intact ulnar nerve function due to compensation by the median nerve.

**✓ Correct Answer: A. Ulnar nerve injury**