A child presents with growth failure. Biochemical analysis shows normal ca2+, normal PTH, reduced phosphate, and increased ALP. What is the diagnosis?

Correct Answer: C. Hypophosphatemic rickets

Hypophosphatemic rickets (X-linked hypophosphatemia, XLH) is the most common form of hereditary rickets in India. The discriminating biochemical pattern here is normal calcium, normal PTH, reduced phosphate, and elevated ALP. This triad—especially the combination of normal PTH with hypophosphatemia—is pathognomonic for hypophosphatemic rickets. In this condition, a defect in the FGF23 pathway (most commonly PHEX gene mutation) leads to excessive renal phosphate wasting and impaired 1,25-dihydroxyvitamin D production. Unlike nutritional rickets or VDDR, PTH remains normal because serum calcium is maintained through compensatory mechanisms. The elevated ALP reflects bone turnover and mineralization defect. Growth failure occurs due to skeletal deformities and metabolic derangement. The normal calcium distinguishes this from VDDR Type 1 (where PTH would be elevated due to hypocalcemia) and Type 2 (where PTH is markedly elevated). This is the classic presentation seen in Indian pediatric orthopedic clinics, often presenting with progressive lower limb deformities and short stature despite adequate nutrition.

Why the other options are wrong

A. Type 2 VDDR — Type 2 VDDR (vitamin D-dependent rickets type 2, 1α-hydroxylase deficiency) presents with elevated PTH due to hypocalcemia, elevated phosphate (not reduced), and severe hypocalcemia. The normal PTH and normal calcium in this case rule out Type 2 VDDR. This is an NBE trap because both are hereditary rickets, but the biochemistry is distinctly different. B. Nutritional rickets — Nutritional rickets (most common in India) presents with elevated PTH as a secondary response to hypocalcemia and hypophosphatemia. The normal PTH in this case excludes nutritional rickets. Additionally, nutritional rickets shows low 25-hydroxyvitamin D levels, whereas hypophosphatemic rickets has normal or elevated 1,25-dihydroxyvitamin D. This is the most common trap—students may default to nutritional rickets without carefully analyzing the PTH level. D. Type 1 VDDR — Type 1 VDDR (vitamin D-dependent rickets type 1, 1α-hydroxylase deficiency) presents with elevated PTH and hypocalcemia. The normal calcium and normal PTH in this case definitively exclude Type 1 VDDR. While both are hereditary, Type 1 VDDR is characterized by severe hypocalcemia-driven secondary hyperparathyroidism, which is absent here.

High-Yield Facts

Mnemonics

VDDR vs Hypophosphatemic Rickets VDDR = High PTH (calcium is low, PTH tries to compensate). Hypophosphatemic = Normal PTH (calcium is normal, no stimulus for PTH). Use this to instantly rule out VDDR types when PTH is normal. XLH Biochemistry: NCP Rule Normal Calcium, Correct (normal) PTH, Phosphatase (ALP) elevated. When you see this triad in a child with rickets, think XLH/hypophosphatemic rickets.

NBE Trap

NBE pairs hereditary rickets (VDDR and hypophosphatemic) to trap students who memorize "hereditary rickets = VDDR" without analyzing the biochemical pattern. The normal PTH is the discriminator that most students miss because they focus on the low phosphate and elevated ALP (which appear in multiple rickets types).

Clinical Pearl

In Indian pediatric practice, a child with progressive genu varum/valgum, short stature, and normal serum calcium should immediately trigger the thought of XLH. Unlike nutritional rickets (which is preventable with vitamin D supplementation), hypophosphatemic rickets requires lifelong phosphate + calcitriol therapy—early diagnosis prevents permanent skeletal deformity and growth stunting.

_Reference: Robbins Ch. 8 (Metabolic Bone Disease); Harrison Ch. 395 (Hypophosphatemic Rickets); OP Ghai Ch. 10 (Rickets)_