## Why option 1 is correct The pattern marked **A** — symmetric bilateral hilar lymphadenopathy with right paratracheal nodes (the "1-2-3 sign" or Garland triad) — is the hallmark of sarcoidosis Scadding stage I. Sarcoidosis is a multisystem granulomatous disorder characterized by non-caseating granulomas. The epithelioid macrophages within these granulomas produce 1α-hydroxylase, the enzyme responsible for converting 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D. Unlike normal renal regulation, this conversion is unregulated and independent of serum calcium and PTH levels, leading to excessive calcitriol production, increased intestinal calcium absorption, and hypercalcemia. The clinical presentation of erythema nodosum + bilateral hilar lymphadenopathy + arthralgias is Löfgren syndrome, a classic acute sarcoidosis presentation. (Harrison 21e Ch 360) ## Why each distractor is wrong - **Option 2 (OAF from malignant lymphocytes)**: This mechanism is seen in lymphoma and other hematologic malignancies, not in sarcoidosis. While sarcoidosis does involve lymphocyte infiltration, the hypercalcemia is driven by macrophage-derived 1α-hydroxylase, not lymphocyte-derived OAF. - **Option 3 (PTHrP from squamous cell carcinoma)**: PTHrP-mediated hypercalcemia is typical of lung cancer and other solid malignancies, not sarcoidosis. The biopsy showed non-caseating granulomas, excluding malignancy. - **Option 4 (Vitamin D intoxication from dietary supplementation)**: This would require exogenous vitamin D intake and malabsorption, neither of which is suggested by the clinical scenario. Sarcoidosis-associated hypercalcemia is endogenous, driven by granuloma macrophage activity. **High-Yield:** Sarcoidosis hypercalcemia = unregulated 1α-hydroxylase in granuloma macrophages → excessive calcitriol → hypercalcemia and hypercalciuria (independent of PTH and serum calcium feedback). [cite: Harrison 21e Ch 360]
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