Froment’s sign is positive due to loss of –
**Core Concept**
Froment's sign is a clinical test used to assess the strength and coordination of the adductor pollicis muscle, which is primarily innervated by the ulnar nerve. The test is based on the principle that the adductor pollicis muscle plays a crucial role in thumb adduction, and its weakness or paralysis can lead to an abnormal movement pattern.
**Why the Correct Answer is Right**
When Froment's sign is positive, it indicates that the patient is using the flexor pollicis longus muscle to compensate for the weakness of the adductor pollicis muscle. This is due to the ulnar nerve damage, which affects the motor fibers that innervate the adductor pollicis muscle. As a result, the patient exhibits a characteristic "pincer grasp" movement, where the thumb flexor muscle contracts to hold a piece of paper or a small object. The flexor pollicis longus muscle is innervated by the anterior interosseous branch of the median nerve, which is unaffected in cases of ulnar nerve damage.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the flexor pollicis longus muscle is not responsible for thumb adduction.
**Option B:** This option is incorrect because the median nerve is not directly involved in Froment's sign.
**Option C:** This option is incorrect because the long flexor muscles of the thumb are not responsible for thumb adduction.
**Clinical Pearl / High-Yield Fact**
Remember that Froment's sign is a sensitive indicator of ulnar nerve damage, but it can also be positive in cases of median nerve damage or other conditions that affect thumb adduction. Always consider the patient's overall clinical presentation and examination findings when interpreting Froment's sign.
**Correct Answer:** D. Ulnar nerve.