A 37-year-old woman came to a clinic complaining of intense pain in her left wrist and a tingling sensation in her left thumb, index and middle fingers, and a part of her ring finger. At first. the pain manifested as irregular pulsing sensation, but by now it has intensified significantly and wakes the woman at night. She has no complaints about her right wrist and fingers. A study of nerve conduction revealed a nerve compression. Which of the nerves is most likely to be compressed in this patient?