All of the following are affected in low radial nerve palsyexcept
## Core Concept
Low radial nerve palsy, also known as radial neuropathy, affects the radial nerve distal to the level of the branching of the posterior interosseous nerve (PIN). This condition primarily impacts the motor and sensory functions innervated by the radial nerve below the point of injury or lesion. The key muscles involved include the extensor muscles of the wrist and fingers.
## Why the Correct Answer is Right
The correct answer, , involves the extensor digitorum communis muscle, which is responsible for wrist extension and is innervated by the radial nerve. In low radial nerve palsy, this muscle is indeed affected, leading to weakness in wrist extension. The radial nerve's superficial branch, which provides sensation to the back of the hand and the first web space, is also affected. However, the muscle primarily responsible for elbow extension, the triceps brachii, is innervated by the radial nerve but above the level of the typical low radial nerve palsy lesion.
## Why Each Wrong Option is Incorrect
* **Option A:** The extensor digitorum communis is affected in low radial nerve palsy, as it is responsible for finger extension at the metacarpophalangeal joints and wrist extension.
* **Option B:** The extensor indicis, involved in finger extension, is also affected as it is innervated by the posterior interosseous nerve (a branch of the radial nerve).
* **Option C:** The brachioradialis muscle, involved in forearm flexion and rotation, is affected as it is innervated by the radial nerve.
* **Option D:** The triceps brachii, primarily responsible for elbow extension, is typically spared in low radial nerve palsy because the lesion usually occurs below the level of its innervation.
## Clinical Pearl / High-Yield Fact
A key clinical feature of low radial nerve palsy is the inability to extend the wrist (wrist drop) and fingers at the metacarpophalangeal joints, but elbow extension remains intact. This condition often results from fractures of the humerus or direct compression injuries to the radial nerve.
## Correct Answer: D. triceps brachii