You are answering a USMLE-style medical question. Question: A 55-year-old man comes to the physician because of a 3-month history of a progressively growing mass in his axilla. During this period, he has had recurrent episodes of low-grade fever lasting for 7 to 10 days, and an 8-kg (18-lb) weight loss. His only medication is a multivitamin. Physical examination shows nontender, right axillary, cervical, and inguinal lymphadenopathy. His serum calcium concentration is 15.1 mg/dL and parathyroid hormone (PTH) concentration is 9 pg/mL. A lymph node biopsy shows granuloma formation and large CD15-positive, CD30-positive cells with bilobed nuclei. Which of the following is the most likely explanation for this patient's laboratory findings? Options: A) Excessive intake of dietary ergocalciferol B) Induction of focal osteolysis by tumor cells C) Secretion of γ-interferon by activated T-lymphocytes D) Production of PTH-related peptide by malignant cells Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.