Hyperphosphatemia (Option B) is the most likely laboratory finding in this child with CKD stage 4 and metaphyseal fraying
The answer is B. Hyperphosphatemia. This child has renal osteodystrophy manifesting as rickets-like changes (metaphyseal fraying), which occurs in the setting of CKD-mineral bone disorder (CKD-MBD). The characteristic laboratory pattern in CKD stage 4 includes hyperphosphatemia as the kidneys can no longer adequately excrete phosphorus when GFR falls below 30 mL/min/1.73 m² 1.
Pathophysiology of CKD-MBD in Stage 4
In CKD stage 4, phosphate retention is the primary metabolic derangement that drives the cascade of mineral bone abnormalities 2, 3:
- Hyperphosphatemia develops when GFR falls below 30 mL/min/1.73 m² as compensatory mechanisms fail 1
- Early phosphate accumulation triggers FGF23 overproduction by bone tissue 2
- Decreased calcitriol (active vitamin D) production by the kidneys occurs 2
- Secondary hyperparathyroidism develops, beginning even earlier at CKD stage 3 (GFR <60 mL/min/1.73 m²) 1
- Hypocalcemia (not hypercalcemia) results from decreased intestinal calcium absorption and skeletal resistance to PTH 2, 3
Expected Laboratory Profile in This Patient
The complete biochemical picture in a child with CKD stage 4 and metaphyseal fraying includes 1:
- Elevated serum phosphorus (>5.5 mg/dL for age) - the hallmark finding
- Elevated PTH (typically >2-3 times upper normal limit) - progressively increasing
- Low or low-normal serum calcium (corrected for albumin)
- Low 25-OH vitamin D (<30 ng/mL) - not high vitamin D
- Elevated alkaline phosphatase - reflecting high bone turnover
Why the Other Options Are Incorrect
Option A (Hypokalemia) is incorrect:
- Hyperkalemia, not hypokalemia, is the concern in advanced CKD 4
- Potassium intake should be limited in children with CKD stages 2-5 who have or are at risk of hyperkalemia 4
Option C (High vitamin D) is incorrect:
- Vitamin D levels are LOW in CKD-MBD, not high 1
- The kidneys lose their ability to convert 25-OH vitamin D to active 1,25(OH)₂D 2
- KDOQI guidelines recommend maintaining 25-OH vitamin D levels above 30 ng/mL, which requires supplementation 1
Option D (High calcium) is incorrect:
- Hypocalcemia, not hypercalcemia, is expected in untreated CKD stage 4 1, 3
- Hypercalcemia only occurs iatrogenically with excessive calcium-based phosphate binders or vitamin D therapy 4
- The guidelines specifically warn about monitoring calcium to avoid hypercalcemia during treatment 1
Clinical Significance and Management
The combination of hyperphosphatemia with metaphyseal fraying indicates inadequate control of CKD-MBD and requires immediate intervention 1:
- Dietary phosphorus restriction to 80% of DRI when both PTH is elevated AND serum phosphorus exceeds normal range 4, 1
- Phosphate binders (calcium-based initially, switching to calcium-free if hypercalcemia develops) 4, 5
- Active vitamin D supplementation 1
- Monitoring serum phosphorus at least every 3 months in CKD stage 4 4, 5
The metaphyseal fraying represents renal osteodystrophy with high bone turnover (osteitis fibrosa) secondary to hyperparathyroidism driven by phosphate retention 6, 2. This differs from nutritional rickets where phosphorus would be low, not high 7.