**Core Concept**
The musculocutaneous nerve is a branch of the brachial plexus that supplies the anterior compartment of the arm, including the biceps brachii, brachialis, and coracobrachialis muscles. It is responsible for elbow flexion, forearm supination, and shoulder adduction.

**Why the Correct Answer is Right**
The musculocutaneous nerve does not innervate the muscles responsible for shoulder flexion. Shoulder flexion is primarily mediated by the suprascapular nerve, which innervates the supraspinatus and infraspinatus muscles, and the axillary nerve, which innervates the deltoid muscle. The musculocutaneous nerve injury at the axilla would result in weakness of elbow flexion, forearm supination, and shoulder adduction, but not loss of flexion of the shoulder.

**Why Each Wrong Option is Incorrect**
**Option B:** Weak elbow flexion is a correct feature of musculocutaneous nerve injury, as the nerve supplies the biceps brachii and brachialis muscles, which are responsible for elbow flexion.
**Option C:** Weak forearm supination is also a correct feature, as the musculocutaneous nerve supplies the biceps brachii muscle, which is responsible for forearm supination.
**Option D:** Loss of sensation on the radial side of the forearm is a correct feature, as the musculocutaneous nerve also supplies the lateral antebrachial cutaneous nerve, which provides sensation to the radial side of the forearm.

**Clinical Pearl / High-Yield Fact**
In musculocutaneous nerve injury, the biceps brachii muscle is often spared due to its dual innervation by the musculocutaneous and femoral nerves. However, the coracobrachialis and brachialis muscles are typically affected, leading to weakness of elbow flexion and forearm supination.

**✓ Correct Answer: A. Loss of flexion of shoulder**