Froment signs is positive in injury of which nerve?
**Core Concept**
The Froment sign is a clinical test used to assess the function of the ulnar nerve at the wrist. It involves asking the patient to hold a piece of paper between their thumb and index finger, with the thumb adducted against resistance. A positive Froment sign indicates weakness or paralysis of the adductor pollicis muscle, which is innervated by the ulnar nerve.
**Why the Correct Answer is Right**
The Froment sign is positive when the patient compensates for the weakness of the adductor pollicis muscle by using the flexor pollicis longus muscle, which is innervated by the median nerve. This compensation is observed as an increased flexion of the interphalangeal joint of the thumb as the patient tries to hold the paper. The ulnar nerve injury results in weakness of the adductor pollicis muscle, which normally opposes the flexor pollicis longus muscle. This leads to an abnormal movement of the thumb, making the Froment sign positive.
**Why Each Wrong Option is Incorrect**
**Option A:** Radial nerve injury would affect the extension of the wrist and fingers, not the adduction of the thumb.
**Option B:** Median nerve injury would affect the flexion of the thumb and fingers, not the adduction of the thumb.
**Option C:** Axillary nerve injury would affect the deltoid and teres minor muscles, not the adductor pollicis muscle.
**Clinical Pearl / High-Yield Fact**
The Froment sign is a useful clinical test to diagnose ulnar nerve injury at the wrist, particularly in patients with a history of trauma or compression of the nerve.
**Correct Answer: C. Ulnar nerve injury.**