Calcium Homeostasis and PTH

A 52-year-old woman with chronic kidney disease (eGFR 28 mL/min/1.73m²) presents with serum calcium 7.2 mg/dL, phosphate 5.8 mg/dL, and PTH 280 pg/mL. Regarding the pathophysiology of secondary hyperparathyroidism in CKD, all of the following are true EXCEPT:

A 52-year-old woman with chronic kidney disease stage 4 presents with serum calcium 7.2 mg/dL, phosphate 5.8 mg/dL, and PTH 420 pg/mL. A 58-year-old man with primary hyperparathyroidism presents with serum calcium 11.8 mg/dL, phosphate 2.1 mg/dL, and PTH 185 pg/mL. Which feature best discriminates secondary hyperparathyroidism (CKD) from primary hyperparathyroidism?

A 48-year-old man with a parathyroid adenoma (primary hyperparathyroidism) is compared with a 55-year-old woman with vitamin D deficiency rickets. Which biochemical finding best distinguishes primary hyperparathyroidism from secondary hyperparathyroidism due to vitamin D deficiency?

A 52-year-old woman from Delhi presents with progressive weakness, bone pain, and recurrent kidney stones over 6 months. She reports nausea and constipation. Laboratory investigations show: serum calcium 11.2 mg/dL (normal 8.5–10.2), serum phosphate 2.8 mg/dL (normal 2.5–4.5), serum creatinine 1.4 mg/dL, 24-hour urinary calcium 380 mg/day (normal <250), and intact PTH 185 pg/mL (normal 10–65). Abdominal ultrasound reveals a 1.2 cm hypoechoic lesion in the lower pole of the right parathyroid gland. What is the primary mechanism responsible for hypercalcemia in this patient?

A 58-year-old man from Bangalore is found to have serum calcium 7.8 mg/dL (normal 8.5–10.2), serum phosphate 5.2 mg/dL (normal 2.5–4.5), and intact PTH 8 pg/mL (normal 10–65) on routine screening. He is asymptomatic. Serum magnesium is 1.6 mg/dL (normal 1.7–2.2). Serum albumin is 4.0 g/dL. Renal function is normal (creatinine 0.9 mg/dL). Vitamin D level is 32 ng/mL (normal >30). What is the most likely diagnosis?