A 64-year-old male noticed the development over the past 3 months of thickened, darkly pigmented skin in axillae and flexural areas of the neck and groin. These areas are neither painful nor pruritic. A punch biopsy of axillary skin reveals undulating epidermal acanthosis with hyperkeratosis and' basal layer hyper pigmentation. Which of the following underlying diseases is he most likely to have?