A 58-year-old man with a 700-pack-peryear smoking history presents to the emergency depament with shoness of breath and hemoptysis. Poable chest radiography demonstrates a large mass centrally located within the left lung field. The serum calcium is 13.0 mg/dL (normal 8.5 to 10.2). The metabolic abnormality described here is likely due to elaboration of which substance