A 68-year-old man presents for evaluation of a skin lesion on the dorsum of his left hand. He worked as a farmer for 40 years and rarely used sunscreen. The lesion has been present for 8 months and has slowly enlarged. He denies itching, pain, or bleeding. Physical examination shows a 1-cm erythematous papule with a rough, “sandpaper-like” yellow-to-brown scale on a sun-exposed area. No lymphadenopathy is noted. Which of the following is the most likely diagnosis?