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lacia include metabolic acidosis, hypercalciuria, hypophosphatemia, and low

1,25(OH) 2 D. Metabolic acidosis leads to impaired bone mineralization and

increased bone resorption. Activation of RANK ligand by systemic acidosis

results in increased bone resorption, which leads to release of calcium, phosphate,

and carbonate in order to counteract metabolic acidosis; however, it results in

hypercalciuria. In addition, acidosis leads to decreased 1α-hydroxylase activity

and consequently, reduced calcium and phosphate absorption from the intestine,

and secondary hyperparathyroidism. Defective tubular reabsorption of phosphate

in proximal RTA also contributes to hypophosphatemia. These alterations in min-

eral homeostasis result in decreased calcium–phosphate solubility product and

impaired mineralization, thereby leading to rickets/osteomalacia (Fig. 5.15).

75. What is the indication of ammonium chloride loading test in a patient with
    RTA?
    Ammonium chloride loading test is indicated in children with a strong clinical
    suspicion of distal RTA but with a normal blood pH. This is usually seen in
    patients who have incomplete or milder forms of distal RTA. The recommended
    dose of ammonium chloride is 0.1 g/Kg body weight administered over a period
    of 1h to avoid gastric irritation, and blood and urine samples are taken every
    hour for 4–6h. After administration of ammonium chloride, it is converted to
    urea in the presence of bicarbonate in the liver, thereby reducing the levels of

a b

R

2 11 10

c

Fig. 5.15 (a) Windswept deformity in a child with rickets due to distal RTA. (b) X-ray of wrist
shows classical features of rickets with metaphyseal fraying, cupping, and splaying. (c) X-ray of
abdomen showing bilateral nephrocalcinosis

5 Rickets–Osteomalacia