Mildly Pruritic Skin Eruption in 44-Year-Old Taiwanese Immigrant

Several years ago, a 44-year-old nonsmoking Taiwanese immigrant to the United States sought medical attention for the skin eruption depicted. This disorder had been present since he was 25; it was mildly pruritic, and involved the scalp, elbows, knees, and dorsal hands (16% of body surface area). He had also noted abnormal fingernails, with onycholysis and pitting having been present for about 5 years. The rest of the physical examination was unremarkable except for mild hypertension (158/92 mm Hg). Pertinent findings from a recent comprehensive laboratory panel included new findings of fasting hyperglycemia and an elevated hemoglobin A1c. He now presents to the dermatologist for a routine semi-annual follow-up visit, his rash well controlled using topical corticosteroids and intermittent narrow-band UVB phototherapy. As incidental complaints, he notes an involuntary 35-pound weight loss and increasing fatigue. He is ultimately referred to the oncologist for further evaluation and care.

The most likely primary site of this patient's malignancy is the...?