Occult Lung Malignancy Presenting With Severe Paraneoplastic Hypercalcemia as an Early Diagnostic Clue in a Patient With Chronic Obstructive Pulmonary Disease (COPD) and Smoking History: A Diagnostic Challenge Mimicking Chronic Infection.

Non-resolving pneumonia in elderly patients with chronic obstructive pulmonary disease (COPD) and a smoking history poses a significant diagnostic challenge, particularly in tuberculosis-endemic regions where infectious causes are often prioritized. We report a 68-year-old male patient with COPD, type 2 diabetes mellitus, and hypertension who presented with progressive dyspnea, evening low-grade fever, anorexia, and significant weight loss over three months. Initial treatment for presumed infective exacerbation of COPD failed despite antibiotics. Extensive workup for tuberculosis, multiple myeloma, and sarcoidosis was negative. Notably, prominent parathyroid hormone (PTH)-independent hypercalcemia was detected early and persisted, serving as the pivotal diagnostic clue. Contrast-enhanced CT (CECT) chest revealed a left lower lobe mass with endobronchial obstruction, and biopsy confirmed squamous cell carcinoma. The hypercalcemia was presumed to be mediated by tumor-derived parathyroid hormone-related peptide (PTHrP). This case highlights the value of recognizing paraneoplastic hypercalcemia as an early red flag for occult lung malignancy in high-risk patients, even with modest smoking exposure, thereby preventing diagnostic delay due to anchoring on infection.