Which of the following muscles is the only muscle capable of extending the metacarpophalangeal (MCP) joint of the digits?

a. Interossei
b. Lumbricales
c. Extensor digitorum
d. Extensor carpi ulnaris

ANS: C
The extensor digitorum is the only muscle of this group that crosses the MCP joint posteriorly and, therefore, is the only muscle that could possibly extend the MCP joint. The interossei and lumbricales cross the MCP anteriorly and lend to MCP joint flexion. The extensor carpi ulnaris does not cross the MCP.