A 40-year-old developed pathological fractures and was found to be having low bone density after a Dexa Scan. Laboratory tests show low plasma calcium, elevated PTH levels, and low vitamin D levels. The patient is on a balanced diet. However, the patient's plasma creatinine and urea are elevated markedly. Which of the following is the most likely reason for the hypocalcemia and reduced bone mass?

Here there are signs of early renal failure. This reduces the levels of 1 alpha-hydroxylase in the proximal tubule cells, thereby reducing the conversion of 25-hydroxyvitamin D, the inactive form of vitamin D, to 1,25-dihydroxyvitamin D, the most active form. The 25-hydroxycholecalciferol is conveed in the cells of the proximal tubules of the kidneys to the more active metabolite 1,25-dihydroxycholecalciferol, which is also called calcitriol or 1,25-(OH)2D3. 1,25-Dihydroxycholecalciferol is also made in the placenta, in keratinocytes in the skin, and in macrophages. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 21. Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.