A 40-yeаr-оld cоmes tо the office to evаluаte pain, tingling, and numbness in the left hand and fingers. The patient reports that the pain worsens at night and now interferes with typing while working as an office secretary. Physical exam of the left hand show a loss of sensation over the dorsal and palmar aspects of the lateral 3 1/2 fingers with weakness of thumb flexion. The affected nerve in the patient was most likely injured at which of the following locations?