A 52-year-old woman has six months of tingling in the right thumb, index and middle fingers, worst at night and relieved by shaking the hand. The muscle bulk at the base of the thumb is reduced and thumb abduction is weak. Her general practitioner wonders whether the median nerve is compressed at the wrist or injured higher up at the elbow.
Which one of the following sensory findings would be expected with a median nerve lesion at the elbow but not with compression at the wrist?