Our understanding of phosphate regulation is changing as newer mediators such as fibroblast growth factor 23 (FGF-23) emerge. Parathyroid hormone (PTH) and phosphate balance is an integral part of chronic disease kidney care. In a teaching case recently published in the American Journal of Kidney Diseases, Leaf and Wolf discuss a physiologic-based approach to the diagnosis of a patient with hyperphosphatemia. The following questions will test your knowledge of phosphate metabolism.

1. A patient with chronic kidney disease stage 3 is noted to have hyperphosphatemia. The regulators of serum phosphate concentration include all of the following except:

A. PTH

B. FGF-23

C. Calcitonin

D. 1,25 vitamin D

2. A 34-year-old man with normal kidney function is noted to have an elevated phosphorus level, and pseudohyperphosphatemia is suspected. The causes of pseudohyperphosphatemia are all of the following except:

A. Hyperlipidemia

B. Lactic acidosis

C. Multiple myeloma

D. Hyperbilirubinemia

3. Reduced PTH levels are seen in all of the following except:

A. Hypomagnesemia

B. Autosomal dominant hypoparathyroidism

C. Hypermagnesemia

D. Pseudohypoparathyroidism

 4. A 20-year-old woman was referred for investigation of hypocalcemia. She reports carpo-pedal spasm and seizures during childhood. Her corrected calcium is 7 mg/dL. Her kidney function is normal, and a spot urine calcium/creatinine ratio is elevated at 1.95 (0.06–0.45). PTH is low normal at 10 pg/mL and serum magnesium is 0.8 mEq/L. Her 2-year-old daughter has also been found to have hypocalcemia. What is the most likely diagnosis?

A. Hypomagnesemia

B. Pseudohypoparathyrodism

C. Autosomal dominant hypoparathyroidism

D. Familial tumoral calcinosis

5. The highest levels of FGF-23 are seen in which of the following conditions?

A. Pseudohyperphosphatemia

B. Hypoparathyroidism

C. Vitamin D toxicity

D. Chronic kidney disease

E. Familial tumoral calcinosis

Post prepared by Dr. Chinmay Patel and Dr. Shailaja Chidella from Hofstra North Shore-LIJ School of Medicine and edited by Dr. Kenar Jhaveri, eAJKD Blog Editor.

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