Brown macules on the lips and nail changes

An elderly man presents with asymptomatic hyperpigmented, confluent, brown macules on his upper and lower lips, along with hyperpigmented macules on his hands. He also has diffuse blue-black bilateral melanonychia with nail dystrophy on his toenails, and his fingernails contain longitudinal melanonychia with hyperpigmentation of the proximal nail beds. He denies any family history of mucocutaneous pigmentation or gastrointestinal or ...