A 59-year-old woman presents with increasing pigmentation of the skin. Physical examination shows hyperkeratosis and hyperpigmentation of the axilla, neck, flexures, and anogenital region. Endocrinologic studies reveal normal serum levels of adrenal coicosteroids and glucocoicoids. If this patient's skin pigmentation represents a paraneoplastic syndrome, the primary tumor would most likely be found in which of the following anatomic locations?