Review Question - QID 213857

A 28-year-old man presents to the emergency department with a laceration to the dorsal radial surface of his left thumb sustained while cutting an avocado. The laceration is approximately 1cm in length and penetrates into the subcutaneous tissue, but there is no exposed bone or tendon. Range of motion is intact, and the patient has numbness distal to the laceration. The physician repairs the laceration primarily in the emergency department with sutures. One year later, the patient presents to his doctor with full range of motion and minimal scarring, but with continued numbness in the dorsal radial aspect of the thumb distal to the laceration. Decreased function in which of the following most likely caused this patient's presentation?