A 56-year-old man with chronic renal insufficiency due to polycystic kidney disease is evaluated for placement of an arteriovenous fistula for dialysis access. Blood pressure is 140/90 mm Hg and pulse is 80/min. Examination shows 2+ bilateral edema of the lower extremities. Estimated glomerular filtration rate is 15 mL/min/1.73 m². Which of the following sets of laboratory findings is most likely in this patient?

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11ec160b_3ced_2b0f_9249_2187c9918415_MD0001_00 This patient has advanced chronic kidney disease (CKD). CKD can cause hyperphosphatemia due to the impaired ability of the kidneys to excrete phosphorus (particularly when GFR is <20 mL/min/1.73 m²). Elevated blood phosphate triggers the release of fibroblast growth factor 23 from bone, which lowers calcitriol (1,25-dihydroxyvitamin D) production and intestinal calcium absorption. In addition, patients with advanced CKD typically have decreased renal conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (the more active form) because of inadequate function of renal tissue. The resulting hypocalcemia, along with hyperphosphatemia, stimulates the secretion of parathyroid hormone (PTH) and leads to secondary hyperparathyroidism. (Choice B) Primary hyperparathyroidism is characterized by hypercalcemia, hypophosphatemia (due to increased renal excretion of phosphorus), and increased renal production of 1,25-dihydroxyvitamin D. In secondary hyperparathyroidism due to CKD, PTH is high but serum phosphate is elevated and hypercalcemia would not be seen. (Choice C) Hypoparathyroidism is characterized by hypocalcemia, hyperphosphatemia, and decreased renal production of 1,25-dihydroxyvitamin D. Hypoparathyroidism is usually caused by autoimmune disease or iatrogenic injury to the parathyroid glands during neck surgery. (Choice D) Vitamin D deficiency (ie, low 25-hydroxyvitamin D) causes decreased absorption of dietary calcium and leads to hypocalcemia. The resulting increase in PTH (secondary hyperparathyroidism) causes decreased renal reabsorption of phosphate, leading to hypophosphatemia. 1,25-Dihydroxyvitamin D levels are typically low, although the increased renal conversion due to PTH may restore levels to within laboratory norms. (Choice E) Elevated 1,25-dihydroxyvitamin D levels can be seen in granulomatous diseases (eg, sarcoidosis) and in excess intake of calcitriol supplements, and lead to increased intestinal absorption of calcium and phosphate (with hypercalcemia and hyperphosphatemia) and suppression of PTH. Educational objective: Chronic kidney disease can cause hyperphosphatemia due to impaired renal excretion of phosphorus. Elevated blood phosphate triggers the release of fibroblast growth factor 23, which lowers calcitriol production and intestinal calcium absorption. The resulting hypocalcemia, along with hyperphosphatemia, leads to secondary hyperparathyroidism. __________ References: Diagnosis, evaluation, prevention, and treatment of chronic kidney disease-mineral and bone disorder: synopsis of the Kidney Disease: Improving Global Outcomes 2017 clinical practice guideline update. (http://www.ncbi.nlm.nih.gov/pubmed/29459980)

A 56-Year-Old Man with Chronic Renal Insufficiency Due to Polycystic