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Difficulty: MediumEndocrine and Metabolic Mechanisms

Match the following clinical and laboratory profiles of disordered mineral and bone metabolism to their primary underlying molecular or cellular mechanism.

  • Asymptomatic hypercalcemia with inappropriately normal parathyroid hormone levels and a fractional excretion of calcium less than 1%.Inactivating mutation of a transmembrane G-protein coupled receptor in the parathyroid gland and renal tubules.
  • Hypocalcemia and hyperphosphatemia with elevated parathyroid hormone levels, accompanied by short stature, round facies, and shortened fourth metacarpals.Maternally inherited post-receptor signaling defect involving the Gs-alpha protein subunit.
  • Normal serum calcium and phosphorus with markedly elevated alkaline phosphatase and a disorganized mosaic pattern of woven and lamellar bone.Primary focal overactivity of multinucleated giant cells followed by chaotic, unregulated bone formation.
  • Severe hypophosphatemia, normal serum calcium, and inappropriately low-normal 1,25-dihydroxyvitamin D levels, presenting with childhood skeletal bowing.Excessive secretion of a phosphatonin leading to downregulation of renal sodium-phosphate cotransporters.

Answer

Asymptomatic hypercalcemia matches with the G-protein coupled receptor mutation (CaSR); Hypocalcemia with skeletal defects matches with the Gs-alpha subunit defect (GNAS1); Normal calcium with elevated ALP matches with multinucleated giant cell overactivity (osteoclasts); Severe hypophosphatemia matches with excessive phosphatonin secretion (FGF23).
Each mineral and bone disorder has a distinct pathophysiology. FHH involves a CaSR (a G-protein coupled receptor) mutation altering the body's calcium thermostat. Pseudohypoparathyroidism involves a post-receptor Gs-alpha defect preventing PTH downstream signaling. Paget disease begins with uncoupled osteoclast overactivity. X-linked hypophosphatemia is driven by FGF23-mediated renal phosphate wasting.

Step-by-Step Solution

1
Analyze the first clinical profile showing hypercalcemia with very low urine calcium and normal PTH.
Identify this syndrome as Familial Hypocalciuric Hypercalcemia (FHH).
The fractional excretion of calcium of less than 1% in the setting of hypercalcemia is the hallmark of FHH, distinguishing it from primary hyperparathyroidism.
2
Analyze the second profile showing hypocalcemia, elevated PTH, and phenotypic skeletal abnormalities.
Identify this as Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy).
Elevated PTH with concurrent hypocalcemia indicates target-organ resistance, and the distinctive skeletal findings point to a maternally inherited GNAS1 mutation.
3
Analyze the third profile showing an isolated elevation in alkaline phosphatase and a mosaic bone pattern.
Identify this as Paget disease of bone.
Normal calcium and phosphorus with highly elevated alkaline phosphatase indicates high bone turnover driven initially by overactive osteoclasts (multinucleated giant cells).
4
Analyze the fourth profile showing severe hypophosphatemia and low-normal 1,25-dihydroxyvitamin D despite the low phosphate.
Identify this as X-linked hypophosphatemic rickets.
Excess FGF23 wastes phosphate in the proximal tubule and inappropriately inhibits the expected rise in 1,25-dihydroxyvitamin D synthesis.

Key Concept

Molecular mechanisms of mineral and bone disorders
Estimated Time:1m 30s
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Topics

Endocrine and Metabolic Mechanisms

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