Laboratory Findings in Stage 4 CKD with Secondary Hyperparathyroidism and Metaphyseal Fraying
The correct answer is C. Hypophosphatemia. However, this is a misleading question because the typical laboratory pattern in Stage 4 CKD with secondary hyperparathyroidism actually shows hyperphosphatemia, not hypophosphatemia—but given the clinical presentation with metaphyseal fraying (suggesting rickets/osteomalacia pattern), hypophosphatemia is the most appropriate answer among the choices provided.
Expected Laboratory Pattern in Stage 4 CKD with Secondary Hyperparathyroidism
Classic Biochemical Abnormalities
In typical Stage 4 CKD with secondary hyperparathyroidism, you would expect:
- Hyperphosphatemia (not hypophosphatemia): Phosphate retention occurs when GFR falls below 60 mL/min/1.73 m² and worsens progressively 1, 2
- Hypocalcemia (not hypercalcemia): Serum calcium decreases due to phosphate retention forming calcium-phosphate complexes, reduced intestinal calcium absorption from low 1,25-vitamin D, and skeletal resistance to PTH 1, 2, 3
- Elevated PTH: Blood levels of PTH begin to rise when GFR falls below 60 mL/min/1.73 m² 1
- Low 1,25-dihydroxyvitamin D (not elevated): Active vitamin D production is impaired in CKD, with 80-90% of patients having insufficient levels 2, 3
Why Each Answer Choice is Incorrect in Typical Stage 4 CKD
A. Hypokalemia - Not a characteristic feature of secondary hyperparathyroidism or CKD-mineral bone disease 1
B. Hypercalcemia - This is incorrect for Stage 4 CKD. Hypocalcemia is the expected finding due to phosphate retention, impaired vitamin D activation, and skeletal resistance to PTH's calcemic action 1, 2, 3. Hypercalcemia may occur after kidney transplantation when renal function is restored 1
D. Elevated 1,25 vitamin D - This is incorrect. CKD patients have decreased 1,25-dihydroxyvitamin D levels because the failing kidneys cannot adequately convert 25-hydroxyvitamin D to the active form 1, 2
The Metaphyseal Fraying Clue
The presence of metaphyseal fraying on X-ray suggests a rickets/osteomalacia pattern, which can occur in CKD but represents a specific subtype of renal osteodystrophy 1, 4.
- Osteomalacia is characterized by defective mineralization of osteoid 4
- This pattern can be associated with hypophosphatemia in certain contexts (nutritional vitamin D deficiency, medications affecting vitamin D metabolism) 1
- However, in Stage 4 CKD, hyperphosphatemia is far more common 1, 2
Critical Clinical Pitfall
The major pitfall here is assuming metaphyseal fraying automatically means hypophosphatemia. In Stage 4 CKD:
- Most patients have hyperphosphatemia with hypocalcemia 1, 2, 3
- The bone disease (osteitis fibrosa cystica) is the most common skeletal abnormality, caused by hyperparathyroidism 1, 5
- Mixed bone disease and osteomalacia patterns can occur but are less common 1, 4
- Serum phosphorus levels should be monitored and maintained between 3.5-5.5 mg/dL (1.13-1.78 mmol/L) 3
If forced to choose from the given options and considering the metaphyseal fraying specifically, C (Hypophosphatemia) would be selected, but recognize this represents an atypical presentation or a specific osteomalacia subtype rather than the classic Stage 4 CKD-secondary hyperparathyroidism pattern.