All of the following are features of musculo – cutaneous nerve injury at axilla except :
The question asks which is NOT a feature of an injury at the axilla. So, if the nerve is injured there, all the muscles it innervates would be affected. The biceps would be paralyzed, leading to loss of elbow flexion and supination. The brachialis would also be affected, contributing to elbow flexion. The coracobrachialis would be paralyzed, causing weakness in arm flexion and adduction.
Sensory loss would occur in the lateral forearm. So, if an option mentions sensory loss in that area, it's a correct feature. However, if an option mentions sensory loss in a different area, like the medial forearm, that's not part of the musculocutaneous nerve's distribution and would be the exception.
Wait, the options are missing, but the correct answer is supposed to be the exception. Common distractors might include involvement of muscles not innervated by the musculocutaneous nerve, like the triceps (innervated by radial nerve), or sensory loss in areas outside the lateral forearm. So, if an option says "loss of sensation in the medial forearm," that's incorrect because that's the medial cutaneous nerve's territory. Similarly, if an option mentions loss of shoulder abduction, that's not correct because the musculocutaneous nerve doesn't innervate the deltoid. The axillary nerve is responsible for that.
Another point: the musculocutaneous nerve is deep in the arm, so injury at the axilla would affect all its branches. Therefore, the absence of the biceps reflex (which is mediated by the musculocutaneous nerve) would be a feature. So if an option says "preserved biceps reflex," that's the exception.
Putting this together, the correct answer would be the option that refers to a feature not associated with musculocutaneous nerve injury. For example, if an option mentions "loss of sensation in the medial forearm," that's incorrect because that's the territory of the medial cutaneous nerve. Alternatively, if an option says "inability to extend the elbow," that's incorrect because elbow extension is controlled by the triceps, which is innervated by the radial nerve.
**Core Concept**
The musculocutaneous nerve innervates the biceps brachii, brachialis, coracobrachialis, and the lateral cutaneous nerve of the forearm. Injury at the axilla disrupts motor function in these muscles and sensory input to the lateral forearm.
**Why the Correct Answer is Right**
An exception to features of musculocutaneous nerve injury would involve a structure not innervated by this nerve. For example, **loss of sensation in the medial forearm** is incorrect because the medial cutaneous nerve of the forearm (C8-T1) mediates this area, not the musculocutaneous nerve