Question 225

Question:

A 16-year-old girl is brought to the emergency department after attempting suicide by cutting her wrist. The deepest part of the wound is between the tendons of the flexor carpi radialis and the flexor digitorum superficialis. This patient is most likely to have a deficit of which of the following?

Correct Answer: E.  Opposition of the thumb and other fingers

This question tests basic anatomical knowledge of the nerves and muscles of the hand and their functions. Such questions are rare on the actual USMLE, but do exist.
The structure between the two tendons mentioned is the median nerve. oss of the median nerve at the wrist results in loss of thenar muscle innervation, resulting in an inability to oppose the thumb.
(A) Adduction and abduction of the fingers are mediated by the palmar and dorsal interosseus muscles of the hand, respectively. These are mostly innervated by the ulnar nerve.
(B) Extension of the index finger requires forearm extensors.
(C) Flexion of fingers 4 and 5 requires forearm flexor muscles. This is a wrist injury.
(D) The ulnar nerve would mediate sensation over the base of the small finger.
(E) Opposition of the thumb and other fingers involves flexion of the flexor carpi radialis (FCR) and flexor digitorum superficialis (FDS), both innervated my the median nerve.
The FDS muscle in the forearm gives rise to 4 tendons that pass through the flexor retinaculum of the carpal tunnel and insert at the proximal interphalangeal joints. (So, it’s an extrinsic muscle of the hand.)
The FCR muscle passes a tendon just lateral to the FC-ulnaris tendon, and inserts at the base of the 2nd metacarpal.