- Type-I vitamin-D-dependent rickets or osteomalacia:
    - failure of conversion of 25-hydroxyvit D to 1,25-dihydroxyvit D is the cause of this disorder;
    - defect is not clearly understood but may be a deficiency or an abnormality of the renal 1-hydroxyvitamin-D-25-hydrolase, believed 
           necessary for the conversion of 1-hydroxyvitamin D to 1,25-Vit D;
           - this is an autosomal recessive disorder;
    - disorder is considered to be a pseudodeficiency state in which patient does not respond to even large doses of orally administered 
           vitamin D, despite a rise in the amount of 25-hydroxyvitamin D generated by the liver;
    - patients may manifest hypocalcemia, hypophosphatemia, and increased alkaline phosphatase;
    - in some cases the defect can be overcome with large doses of vitamin D, but more importantly it can be completely eliminated by 
           physiological doses of 1,25-dihydroxyvitamin D;

- Type-II vitamin-D-dependent rickets or osteomalacia:
    - this is believed to represent an end-organ insensitivity to autogenous 1,25-dihydroxyvitamin D;
    - pathogenesis is obscure but the cause appears to be a genetic error that results in the failure of the gut (and other) cells to recognize 
          autogenous 1,25-dihydroxyvitamin D;
           - this is an autosomal recessive disorder;
    - patients become rachitic despite normal or high serum concentrations of 25-hydroxyvitamin D & 1,25-dihydroxyvitamin D & remain 
            rachitic despite high doses of vitamin D or calcitriol in diet;
    - this syndrome may share similarities to hypophosphatemic rickets with alopecia in which the nuclear receptor for 1,25-dihydroxyvitamin D 
           in the gut cell is genetically altered;
    - patients may manifest hypocalcemia;
    - these patients may respond to exogenously administered synthetic 1,25-dihydroxyvit D but may need additional calcium administered 
           orally or by infusion for the control of symptoms