A 21-year-old woman comes to the emergency department after she was struck in the arm by a line drive during a softball game. Physical examination shows weakness on abduction and extension of the thumb. There is also weakness on extension of the wrist and metacarpophalangeal (MCP) joints. An x-ray of the arm shows no abnormalities. An MRI shows soft tissue injury to the central aspect of the lateral humerus. Which of the following muscles is most likely spared from injury?

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