A 55-year-old man has had a foot ulcer for 2 months that has not healed. Physical examination shows a 2-cm shallow, nonhealing ulceration of the left medial malleolus. There is a symmetric decreased sensation in the distal regions of the lower extremities. He has a history of multiple urinary tract infections resulting from difficulty in completely emptying the bladder. He is impotent. Which of the following pathologic findings is most likely to be present in the peripheral nerves?

The features described are consistent with peripheral neuropathy associated with diabetes mellitus. Both motor and sensory nerves can be involved, and there may be an autonomic neuropathy. Histologic examination shows an axonal neuropathy with segmental demyelination. Difficulty in emptying the urinary bladder and impotence are results of autonomic neuropathy. Longer nerves are affected first; this explains the lower leg involvement and accounts for many cases of the diabetic foot, with trauma and subsequent ulceration. Acute inflammation is not generally seen in neuropathies. Lymphocytic infiltrates may be seen in Guillain-Barre syndrome. Onion bulb formation is a feature of the hereditary neuropathy known as Refsum disease. Wallerian degeneration typically occurs with traumatic transection of a nerve.