Match the following diagnostic laboratory profiles of calcium and phosphate homeostasis to… — USMLE Step 2 CK

Match the following diagnostic laboratory profiles of calcium and phosphate homeostasis to their most likely underlying pathophysiologic etiology. Assume all patients have normal serum albumin and normal magnesium levels unless otherwise indicated.

$\text{Ca}^{2+} \uparrow$ , $\text{PO}_4^{3-} \downarrow$ , $\text{PTH} \uparrow$ , $\text{24-hr urine Ca}^{2+} \uparrow$ Primary hyperparathyroidism
$\text{Ca}^{2+} \uparrow$ , $\text{PO}_4^{3-} \leftrightarrow$ , $\text{PTH} \leftrightarrow \text{or} \uparrow$ , $\text{urine calcium-to-creatinine clearance ratio} < 0.01$ Familial hypocalciuric hypercalcemia
$\text{Ca}^{2+} \downarrow$ , $\text{PO}_4^{3-} \downarrow$ , $\text{PTH} \uparrow$ , $\text{25(OH)D} \downarrow$ , $\text{1,25(OH)}_2\text{D} \downarrow$ Severe dietary vitamin D deficiency
$\text{Ca}^{2+} \downarrow$ , $\text{PO}_4^{3-} \uparrow$ , $\text{PTH} \uparrow$ , $\text{1,25(OH)}_2\text{D} \downarrow$ , $\text{BUN}/\text{Cr} \uparrow$ End-stage renal disease
$\text{Ca}^{2+} \uparrow$ , $\text{PO}_4^{3-} \leftrightarrow \text{or} \uparrow$ , $\text{PTH} \downarrow$ , $\text{1,25(OH)}_2\text{D} \uparrow$ , $\text{25(OH)D} \leftrightarrow$ Granulomatous disease (e.g., sarcoidosis)

Answer

1 matches Primary hyperparathyroidism, 2 matches Familial hypocalciuric hypercalcemia, 3 matches Severe dietary vitamin D deficiency, 4 matches End-stage renal disease, 5 matches Granulomatous disease.

Accurate diagnosis of calcium disorders requires interpreting the directionality of PTH relative to calcium, assessing renal handling of these electrolytes, and recognizing the different active forms of vitamin D. Identifying the etiology dictates appropriate management (e.g., surgery for primary hyperparathyroidism vs. observation for FHH).

Step-by-Step Solution

Analyze the serum calcium and PTH levels as the primary diagnostic branch point.

Hypercalcemia with high/normal PTH indicates PTH-dependent hypercalcemia (profiles 1 and 2). Hypercalcemia with low PTH indicates PTH-independent hypercalcemia (profile 5). Hypocalcemia with high PTH indicates secondary hyperparathyroidism (profiles 3 and 4).

PTH is the primary regulator of serum calcium; its response defines the core pathophysiologic category.

Evaluate urine calcium excretion in patients with PTH-dependent hypercalcemia.

Profile 1 shows high urine calcium (Primary hyperparathyroidism), whereas Profile 2 shows a calcium-to-creatinine clearance ratio < 0.01 (Familial hypocalciuric hypercalcemia).

FHH mimics mild primary hyperparathyroidism but is distinguished by profound renal calcium conservation due to CaSR mutation.

Examine renal function, phosphorus, and vitamin D levels in hypocalcemic patients.

Profile 4 has elevated BUN/Cr and hyperphosphatemia (End-stage renal disease). Profile 3 has normal renal function, hypophosphatemia, and low 25(OH)D (Dietary vitamin D deficiency).

Kidneys excrete phosphate; renal failure causes phosphate retention, whereas pure vitamin D deficiency decreases intestinal phosphate absorption and triggers phosphaturic secondary hyperparathyroidism.

Investigate the mechanism of PTH-independent hypercalcemia in Profile 5.

Suppressed PTH with elevated 1,25(OH)2D but normal 25(OH)D points to extrarenal 1-alpha-hydroxylase activity.

Granulomatous diseases like sarcoidosis produce active vitamin D independently of PTH regulation, causing hypercalcemia and suppressing endogenous PTH.

Key Concept

Differential diagnosis of calcium and phosphate disorders

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