Ans, is 'b' i.e., Vitamin D deficiencyOsteomalaciao While rickets affect immature skeleton, osteomalacia affects mature skeleton. Osteomalacia, which means softening of bones, is the adult counterpart of rickets. It is primarily due to deficiency of vitamin D. This results in failure to replace the turnover of calcium and phosphorus in the organic matrix of bone. Hence, the bone content is demineralised and the bony substance is replaced by soft osteoid tissue. The osteoid is increased at the cost of mineralized bone.Causes of osteomalaciao Lack of exposure to sunlight (indoor stay )o Dietary deficiency of vitamin Do Under-nutrition during pregnancyo Malabsorption syndrome, partial gastrectomy and all causes of malabsorption,o Derangement of vitamin D and phosphorus metabolism (hereditary or acquired),o Anticonvulsant therapy : - Phenytoin.o Renal failure : Renal osteodystrophy.o Most common cause of osteomalacia is deficiency of vitamin D.Clinical features of osteomalaciao Clinical features of osteomalacia include: -Bone pains : Skeletal discomfort ranging from backache to diffuse bone pains may occur. Bone tenderness is common.Muscular weakness : The patient feels very weak. He may have difficulty in climbing up and down the stairs. A waddling gait is not unusual. Tetany may manifest as carpopedal spasm and facial twitching.Spontaneous fractures occurs usually in spine, and may result in kyphosis.Radiological feature of osteomalaciao X-ray finding in osteomalacia are : -i) Diffuse rarefaction of bonesii) Looser's zone (pseudo-fractures) Radiolucent zones at the sites of stress.Triradiate pelvis in femalesProtrusio-acetabuli, i.e., the acetabulum protruding into the pelvis.Serum biochemistry in osteomalaciao Serum calcium is mildly decreased or normal,o Serum phosphate is lowo Alkaline phosphatase is raisedo Parathormone level is increased