**Question:** In a vehicle accident, the musculocutaneous nerve was completely severed, but still the person was able to weekly flex the elbow joint. All of the following muscles are responsible for this flexion, EXCEPT?

A. Biceps brachii
B. Brachialis
C. Brachioradialis
D. Triceps brachii

**Core Concept:**
The musculocutaneous nerve (C8, T1) is a trunk of the brachial plexus that supplies motor innervation to the lateral cutaneous nerve of the arm, the coracobrachialis, and the majority of the biceps brachii muscle. The other muscles responsible for elbow flexion, brachialis, brachioradialis, and triceps brachii, are innervated by the radial nerve (C5, C6, C7, C8, T1).

**Why the Correct Answer is Right:**
The correct answer is D, Triceps brachii, because it is innervated by the radial nerve, which is not affected in this scenario where the musculocutaneous nerve is severed. In contrast, A, B, and C are innervated by the musculocutaneous nerve, which is the reason why they are still able to contract despite the nerve injury.

**Why Each Wrong Option is Incorrect:**
A. Biceps brachii: This muscle is innervated by the musculocutaneous nerve and remains functional despite the injury, allowing for elbow flexion.
B. Brachialis: This muscle is also innervated by the musculocutaneous nerve, which means it remains functional and can contribute to elbow flexion.
C. Brachioradialis: Similar to A and B, this muscle is innervated by the musculocutaneous nerve, allowing it to function despite the nerve injury.

**Clinical Pearl:**
In situations where a nerve is severed, the patient may still experience muscle function due to collateral pathways and/or reinnervation by other nerves. This is referred to as the "brachial plexus bridge," which allows for some muscle function to persist despite a nerve injury. This concept is relevant for understanding the recovery of function after nerve injuries and the role of collateral pathways in the human body.