A 24-year-old woman has had episodes of numbness and tingling in both hands for 5 months. The problem is worse near the end of the day and makes it difficult for her to use the computer keyboard. The thumb and first two fingers are most affected. There is no pain or swelling, and she does not recall any trauma to the upper extremities. On physical examination, she has a positive Tinel sign and decreased sensation to light touch and pinprick over the palmar surface of both hands in the distribution of the first three digits. Thenar muscle atrophy is present. This neuropathy is most likely due to which of the following underlying causes?