A 48-year-old man with long-standing type 2 diabetes mellitus (recent glycated hemoglobin level, 6.5%) and chronic kidney disease (baseline creatinine level, 3.3 mg per deciliter [292 μmol per liter]; glomerular filtration rate, 24 ml per minute per 1.73 m2 of body-surface area) presented to his primary care physician with a 3-month history of numbness, tingling, and faint violaceous discoloration of the tips of multiple fingers and toes. His physical examination showed reduced light-touch sensation in a glove-and-stocking distribution; the radial and pedal pulses were palpable. The vitamin B12 level was 260 pg per milliliter (192 pmol per liter; normal range, 190 to 950 pg per milliliter [140 to 701 pmol per liter]). He did not smoke tobacco, drink alcohol, or use illicit drugs. One month later, a nontraumatic wound developed on the left foot. The ankle–brachial index (ABI) was 1.2 on both sides (normal range, 0.91 to 1.3). Wound care was initiated for a presumed neuropathic ulcer.