A patient with which of the following disorders is at high risk for developing hyperphosphatemia?
A. Hyperkalemia
B. Hyponatremia
C. Hypocalcemia
D. Hyperglycemia
Correct Answer: C. Hypocalcemia
Because calcium and phosphorus ratios are inversely proportional, when phosphorus levels are high, calcium levels are low. Phosphate binds calcium avidly, causing acute hypocalcemia. Acute hypocalcemia secondary to hyperphosphatemia may also result from renal failure or excess tissue breakdown because of rhabdomyolysis or tumor lysis. In acute hyperphosphatemia, calcium is deposited mostly in the bone but also in the extraskeletal tissue.
Option A: In contrast, in chronic hyperphosphatemia, which is nearly always from chronic renal failure, calcium efflux from the bone is inhibited and the calcium absorption is low, because of reduced renal synthesis of 1,25-dihydroxyvitamin D.
Option B: Several sodium-coupled transport proteins have been identified that enable intracellular uptake of phosphate by taking advantage of the steep extracellular-to-intracellular sodium gradient. Type 1 sodium phosphate cotransporters are expressed predominantly in kidney cells on the apical membranes of proximal tubule cells and liver.
Option D: Phosphorus homeostasis is normally maintained through several mechanisms. GI absorption must be matched by renal excretion, and cellular release is balanced by uptake in other tissues. Hyperphosphatemia occurs when the phosphorus load (from GI absorption, exogenous administration, or cellular release) exceeds renal excretion and tissue uptake.
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