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Hypercalcemia in lung cancer is often associated with the secretion of Parathyroid Hormone-related Peptide (PTHrP) by the tumor. PTHrP mimics the action of parathyroid hormone and stimulates bone resorption, leading to an increase in calcium levels in the blood. This paraneoplastic syndrome is commonly seen in certain types of lung cancers, most notably squamous cell carcinoma, but can also occur in other lung malignancies.

The other options do not accurately capture the relationship between hypercalcemia and lung cancer. While renal failure can lead to hypercalcemia, it is not the sole cause in the context of lung cancer-related hypercalcemia. Hypercalcemia can be present in patients regardless of their treatment status, so it does not occur exclusively in treated patients. Furthermore, while hypercalcemia can be associated with various lung cancers, it is not an indicator that necessarily points to a high likelihood of small cell lung cancer specifically. Thus, the secretion of PTHrP is the primary mechanism explaining this paraneoplastic phenomenon in lung cancer.