## Why Squamous cell carcinoma with PTHrP-mediated hypercalcemia is right Squamous cell carcinoma is the classic central/hilar lung malignancy causing endobronchial obstruction and atelectasis in heavy smokers. The Golden S sign (concave inferior border of collapsed lobe meeting the convex mass contour) is pathognomonic for central mass-induced lobar collapse. Squamous cell carcinoma characteristically presents as a central mass, causes hemoptysis due to endobronchial growth, cavitates, and frequently produces parathyroid hormone-related protein (PTHrP), leading to hypercalcemia of malignancy — a paraneoplastic syndrome unique to this histology among the options. The 50 pack-year smoking history is the strongest epidemiologic link to squamous cell carcinoma. ## Why each distractor is wrong - **Adenocarcinoma with peripheral location and late airway involvement**: Adenocarcinoma is the most common lung cancer overall but typically arises in the lung periphery (not central/hilar) and is much less likely to cause endobronchial obstruction and acute atelectasis. It is not strongly associated with smoking as a primary risk factor. - **Large cell carcinoma with rapid necrosis and cavitation**: Although large cell carcinoma can cavitate, it is rare (< 5% of lung cancers), not strongly linked to smoking, and does not classically present as a central endobronchial mass causing the Golden S sign. It is not the expected answer for this clinical-radiologic pattern. - **Bronchial carcinoid with slow growth and minimal smoking association**: Bronchial carcinoids are low-grade neuroendocrine tumors that grow slowly, are not associated with smoking, and while they can cause airway obstruction, they do not present with hemoptysis, weight loss, and acute atelectasis in a 65-year-old heavy smoker. **High-Yield:** Squamous cell carcinoma = central/hilar + smoking + hemoptysis + cavitation + PTHrP hypercalcemia; adenocarcinoma = peripheral + non-smokers/light smokers. [cite: Felson Principles of Chest Roentgenology; Robbins 10e — Lung Tumors]
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