Froment’s sign is positive in cases of weakness of –
**Core Concept**
Froment's sign is a clinical test used to assess the strength of the adductor pollicis muscle and the integrity of the ulnar nerve. It involves asking the patient to hold a piece of paper between their thumb and index finger and then resist the examiner's attempt to pull the paper away.
**Why the Correct Answer is Right**
Froment's sign is positive when the patient uses their flexor pollicis longus muscle, which is innervated by the anterior interosseous nerve, a branch of the median nerve, to adduct the thumb in place of the weakened adductor pollicis muscle, which is innervated by the ulnar nerve. This indicates ulnar nerve palsy or weakness of the adductor pollicis muscle. The ulnar nerve is responsible for innervating the adductor pollicis muscle, and damage to this nerve can lead to weakness of this muscle.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Froment's sign is not specifically related to the median nerve, which is responsible for the flexor pollicis longus muscle, but rather the ulnar nerve, which is responsible for the adductor pollicis muscle.
**Option B:** This option is incorrect because Froment's sign is not related to the radial nerve, which is responsible for the extensor muscles of the forearm and hand, but rather the ulnar nerve.
**Option C:** This option is incorrect because Froment's sign is not specifically related to the brachial plexus, which is a complex network of nerves that supply the arm and hand, but rather the ulnar nerve.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that Froment's sign is a clinical test that can help diagnose ulnar nerve palsy or weakness of the adductor pollicis muscle, and it's often used in conjunction with other clinical tests to confirm the diagnosis.
**Correct Answer:** C.