**Core Concept**
The tourniquet test is a clinical examination technique used to assess the circulation and nerve function in the limbs. It involves applying a tourniquet to the limb and observing the patient's response to pain or numbness.

**Why the Correct Answer is Right**
The tourniquet test is a function of the median nerve. When a tourniquet is applied to the upper limb, it compresses the radial and median nerves, causing numbness and tingling in the distribution of these nerves. The median nerve is responsible for innervating the palmar surface of the hand, including the thumb, index, and middle fingers. Compression of this nerve can lead to a characteristic "claw hand" deformity.

**Why Each Wrong Option is Incorrect**
**Option A:** The radial nerve is responsible for wrist extension and is not the primary nerve affected by the tourniquet test.
**Option B:** The ulnar nerve is responsible for innervating the little finger and ring finger, and is not the primary nerve affected by the tourniquet test.
**Option C:** The musculocutaneous nerve is responsible for innervating the biceps and brachialis muscles, and is not the primary nerve affected by the tourniquet test.

**Clinical Pearl / High-Yield Fact**
The tourniquet test is a useful clinical examination technique for diagnosing nerve compression syndromes, such as carpal tunnel syndrome and cubital tunnel syndrome.

**Correct Answer: D. Median nerve.**