**Core Concept**
Paraneoplastic syndromes occur when tumors secrete hormones or bioactive substances that mimic endocrine functions, leading to systemic effects unrelated to the tumor’s direct invasion. In lung cancer, particularly squamous cell carcinoma, ectopic production of parathyroid hormone-related peptide (PTHrP) is a common cause of hypercalcemia.

**Why the Correct Answer is Right**
Squamous cell carcinomas of the lung frequently secrete PTHrP, which acts on bone and kidney to increase calcium levels. PTHrP binds to the same receptor as parathyroid hormone (PTH), stimulating osteoclast activity and enhancing calcium reabsorption in the kidneys. This results in hypercalcemia, even in the absence of primary hyperparathyroidism. A serum calcium of 11.5 mg/dL is elevated (normal: 8.5–10.5 mg/dL), consistent with PTHrP-induced hypercalcemia.

**Why Each Wrong Option is Incorrect**
Option A: Parathyroid hormone is not produced by lung tumors and is not associated with ectopic secretion in this context. Its presence would require primary parathyroid disease, which is ruled out by the absence of bone metastases and no other parathyroid signs.
Option B: Calcitonin lowers serum calcium by inhibiting bone resorption and increasing calcium excretion; it causes hypocalcemia, not hypercalcemia. It is not secreted by lung tumors and would not explain elevated calcium.
Option D: Calcitonin-related peptide (CRP) is a neuropeptide involved in pain and inflammation, not calcium regulation, and does not cause hypercalcemia.

**Clinical Pearl / High-Yield Fact**
Lung squamous cell carcinoma is a classic source of ectopic PTHrP, leading to hypercalcemia. The presence of elevated calcium with no bone metastases should prompt consideration of paraneoplastic hypercalcemia due to PTHrP. This is a hallmark of paraneoplastic syndromes in thoracic malignancies.

✓ Correct Answer: C. Parathyroid hormone-related peptide