Vitamin D-dependent rickets type I Rickets can be due to resistance to the action of vitamin D as well as to vitamin D deficiency. Vitamin D-dependent rickets type I, previously termed pseudovitamin D-resistant rickets , differs from true vitamin D-resistant rickets (vitamin D-dependent rickets type II, see below) in that it is less severe and the biochemical and radiographic abnormalities can be reversed with appropriate doses of the vitamin's active metabolite, 1,25(OH) 2 D 3 . Physiologic amounts of calcitriol cure the disease This finding fits with the pathophysiology of the disorder, which is autosomal recessive, and is now known to be caused by mutations in the gene encoding 25(OH)D-1a-hydroxylase. Both alleles are inactivated in all patients, and compound heterozygotes, harboring distinct mutations, are common. Clinical features include hypocalcemia, often with tetany or convulsions, hypophosphatemia, secondary hyperparathyroidism, and osteomalacia, often associated with skeletal deformities and increased alkaline phosphatase. Treatment involves physiologic replacement doses of 1,25(OH) 2 D 3 Vitamin D-dependent rickets type II Vitamin D-dependent rickets type II results from end-organ resistance to the active metabolite 1,25(OH) 2 D 3 . The clinical features resemble those of the type I disorder and include hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and rickets but also paial or total alopecia. Plasma levels of 1,25(OH) 2 D are elevated, in keeping with the refractoriness of the end organs. This disorder is caused by mutations in the gene encoding the vitamin D receptor; treatment is difficult and requires regular, usually nocturnal calcium infusions (harrison 18 pg 3117)