The adult form of RICKETS. It is due to inadequate mineralisation of bony tissue caused by a deficiency of vitamin D, and may arise because of inadequate intake or to impaired absorption such as in MALABSORPTION SYNDROME. It may also be due to renal disease, as the kidney is responsible for the hydroxylation of cholecalciferol, which has virtually no metabolic action, to dihydroxy-cholecalciferol – the metabolically active form of the vitamin. (See APPENDIX 5: VITAMINS.)