Match each constellation of laboratory findings to the most likely underlying pathophysiol… — USMLE Step 2 CK

Match each constellation of laboratory findings to the most likely underlying pathophysiologic diagnosis.

Calcium $11.2\text{ mg/dL}$ , Phosphorus $2.2\text{ mg/dL}$ , Intact PTH $125\text{ pg/mL}$ , 24-hour urine calcium $350\text{ mg/day}$ Primary hyperparathyroidism
Calcium $7.6\text{ mg/dL}$ , Phosphorus $5.8\text{ mg/dL}$ , Intact PTH $450\text{ pg/mL}$ , $1,25$ -dihydroxyvitamin D $12\text{ pg/mL}$ , Creatinine $3.8\text{ mg/dL}$ Chronic kidney disease-mineral and bone disorder
Calcium $12.8\text{ mg/dL}$ , Phosphorus $2.1\text{ mg/dL}$ , Intact PTH $8\text{ pg/mL}$ , PTH-related peptide $85\text{ pg/mL}$ Humoral hypercalcemia of malignancy
Calcium $10.9\text{ mg/dL}$ , Phosphorus $2.5\text{ mg/dL}$ , Intact PTH $75\text{ pg/mL}$ , 24-hour urine calcium $45\text{ mg/day}$ Familial hypocalciuric hypercalcemia
Calcium $8.1\text{ mg/dL}$ , Phosphorus $2.1\text{ mg/dL}$ , Intact PTH $210\text{ pg/mL}$ , 25-hydroxyvitamin D $8\text{ ng/mL}$ Severe nutritional vitamin D deficiency

Answer

The laboratory patterns uniquely correspond to autonomous PTH production (Primary hyperparathyroidism), renal failure with impaired vitamin D activation (CKD-MBD), ectopic PTHrP production (Humoral hypercalcemia of malignancy), altered calcium sensing (Familial hypocalciuric hypercalcemia), and deficient substrate for active vitamin D (Severe nutritional vitamin D deficiency).

Each specific combination of serum calcium, phosphorus, PTH, and supplementary markers uniquely identifies the locus of pathology in calcium homeostasis. High calcium with high PTH and hypercalciuria is the hallmark of primary hyperparathyroidism. Conversely, high calcium with high PTH but hypocalciuria defines FHH. High calcium with suppressed PTH but elevated PTHrP confirms HHM. In hypocalcemic states, a high phosphorus indicates renal failure (CKD-MBD), whereas a low phosphorus with low 25-hydroxyvitamin D confirms nutritional vitamin D deficiency.

Step-by-Step Solution

Evaluate the serum calcium and intact PTH levels to categorize the primary disturbance.

Identifies PTH-dependent hypercalcemia (high Ca, high PTH), PTH-independent hypercalcemia (high Ca, low PTH), or secondary hyperparathyroidism (low/normal Ca, high PTH).

This fundamental division narrows the differential diagnosis immediately.

Analyze phosphorus levels in the context of the PTH-calcium axis.

Low phosphorus with high PTH confirms PTH-mediated phosphate wasting (Primary hyperparathyroidism, Vitamin D deficiency). High phosphorus with high PTH suggests impaired renal excretion (CKD-MBD).

Phosphorus homeostasis differentiates primary renal failure from isolated PTH or Vitamin D abnormalities.

Utilize specific secondary markers (24-hour urine calcium, PTHrP, 25-OH Vitamin D) to confirm the exact etiology.

Differentiates Primary hyperparathyroidism (high urine Ca) from FHH (low urine Ca); confirms HHM via PTHrP; and distinguishes Vitamin D deficiency from CKD.

These specific markers isolate the pathophysiological root cause when the primary calcium-PTH axis shows similar patterns.

Key Concept

Interpretation of the calcium, phosphorus, parathyroid hormone, and vitamin D axes to diagnose specific metabolic bone and mineral disorders.

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