✅ Model Answer:
• Diagnosis: X-linked hypophosphatemic rickets (XLH – PHEX mutation).
• Evidence: Hypophosphatemia (2.0 mg/dL), normal calcium and PTH, normal 25-OH vitamin D, elevated alkaline phosphatase (rickets), short stature, bowed legs, waddling gait, bone pain.
• Next step: Genetic testing (PHEX). Start oral phosphate supplementation (20-40 mg/kg/day in divided doses) + calcitriol (0.01-0.1 mcg/kg/day). For refractory cases, burosumab (anti-FGF-23 antibody). Monitor growth, bone pain, and serum phosphate/calcium.

✅ Model Answer:
• Gene: PHEX (phosphate-regulating endopeptidase homolog, X-linked) located on chromosome Xp22.1.
• Inheritance: X-linked dominant (males are more severely affected; females may have milder phenotype due to X-inactivation).
• Prevalence: 1 in 20,000 (most common inherited cause of rickets).
• Pathophysiology: Loss-of-function mutations in PHEX → increased circulating FGF-23 (fibroblast growth factor 23) → FGF-23 inhibits renal phosphate reabsorption (by downregulating NaPi-IIa cotransporters) and suppresses 1α-hydroxylase → hypophosphatemia and low 1,25-(OH)₂D → impaired bone mineralization → rickets/osteomalacia.
• Other genes: FGF23, DMP1, ENPP1, FAM20C (causing similar phenotypes – autosomal dominant hypophosphatemic rickets, ADHR).

✅ Model Answer:
• Skeletal features:
- Bowed legs (genu varum).
- Short stature.
- Waddling gait (proximal muscle weakness due to osteomalacia).
- Bone pain.
- Widening of wrists and ankles.
- Delayed walking.
- Dental abscesses (due to defective dentin mineralization).
- Rachitic rosary, Harrison sulcus.
• Biochemical:
- Hypophosphatemia (low serum phosphate).
- Normal serum calcium.
- Normal or low PTH.
- Normal 25-OH vitamin D.
- Low 1,25-(OH)₂D (calcitriol).
- Elevated alkaline phosphatase (ALP) – bone turnover.
- Elevated FGF-23.
- Phosphaturia (low TmP/GFR).
• Onset: Usually presents in early childhood (2-5 years).

✅ Model Answer:
• Biochemical:
- Serum phosphate: Low.
- Serum calcium: Normal.
- PTH: Normal or low (if elevated, think of secondary hyperparathyroidism).
- 25-OH vitamin D: Normal (distinguishes from nutritional rickets).
- 1,25-(OH)₂D: Low (due to FGF-23 suppression of 1α-hydroxylase).
- Alkaline phosphatase: Elevated (bone turnover).
- FGF-23: Elevated (confirmatory).
- Urine TmP/GFR: Low (renal phosphate wasting).
• Imaging: X-rays of wrists/knees – rachitic changes (cupping, fraying, widening of metaphyses), bowing of long bones.
• Genetic testing: PHEX gene sequencing (confirmatory).
• Family history: X-linked dominant pattern.

✅ Model Answer:
• Oral phosphate supplementation:
- Dose: 20-40 mg/kg/day of elemental phosphorus (in 3-4 divided doses).
- Goal: Improve serum phosphate and bone mineralization.
- Risk: Hypercalcemia, nephrocalcinosis (if vitamin D is given without phosphate).
• Calcitriol (1,25-(OH)₂D):
- Dose: 0.01-0.1 mcg/kg/day.
- Goal: Increase intestinal calcium and phosphate absorption.
- Monitor: Serum calcium, phosphate, PTH, urine calcium/creatinine.
• Burosumab (Crysvita):
- Anti-FGF-23 monoclonal antibody: For refractory cases.
- Dose: 0.4-1.0 mg/kg SC every 2 weeks.
- Indication: Improves phosphate levels, bone healing, and growth.
• Growth hormone: May be considered for short stature.
• Orthopedic: Surgery for severe bowing.
• Monitor: Growth, bone pain, serum phosphate, calcium, ALP, and renal function.

✅ Model Answer:
• Skeletal:
- Severe bowing of legs (genu varum/valgum).
- Short stature (often in the lowest percentile).
- Osteomalacia (bone pain, fractures).
- Spinal stenosis (in adults).
- Enthesopathy (calcification of tendons/ligaments).
• Dental: Dental abscesses, enamel defects.
• Treatment-related:
- Hypercalcemia, hypercalciuria (if calcitriol is overdosed).
- Nephrocalcinosis (due to calcium-phosphate product).
- Secondary hyperparathyroidism (if phosphate is given without calcitriol).
• Psychosocial: Impact of short stature, chronic pain, and recurrent treatments.
• Growth failure.

✅ Model Answer:
• Prognosis:
- Good with early and appropriate treatment.
- Growth: Can be improved with phosphate/calcitriol or burosumab.
- Skeletal deformities: Can be corrected with treatment; surgery may be needed for severe bowing.
- Quality of life: Good with multidisciplinary care.
- Life expectancy: Normal.
- Burosumab: Improves phosphate levels and bone healing significantly.
• Long-term follow-up:
- Monitor growth.
- Monitor serum phosphate, calcium, PTH, ALP.
- Monitor renal function and urine calcium.
- Monitor for nephrocalcinosis (renal ultrasound).
- Monitor for dental complications.
- Genetic counseling.

✅ Model Answer:
• XLH:
- Serum calcium: Normal.
- Serum phosphate: Low.
- PTH: Normal or low.
- 25-OH vitamin D: Normal.
- 1,25-(OH)₂D: Low (due to FGF-23).
- ALP: Elevated.
- FGF-23: Elevated.
- Genetics: X-linked (PHEX mutation).
- Treatment: Phosphate + calcitriol (or burosumab).
- Response: Slowly responds to treatment.
• Nutritional rickets (vitamin D deficiency):
- Serum calcium: Low or normal.
- Serum phosphate: Low (due to secondary hyperparathyroidism).
- PTH: Elevated (secondary hyperparathyroidism).
- 25-OH vitamin D: Low.
- 1,25-(OH)₂D: Normal or low (depending on vitamin D status).
- ALP: Elevated.
- FGF-23: Normal.
- Genetics: Not inherited.
- Treatment: Vitamin D + calcium supplementation.
- Response: Rapid improvement.