MRCP-0349

MRCP-0349

A 30-year-old man presented with polydipsia and polyuria for the last two years. Investigations reveal: Serum urea 9.5 mmol/L (2.5-7.5), Serum creatinine 108 mol/L (60-110), Serum corrected calcium 2.9 mmol/L (2.2-2.6), Serum phosphate 0.7 mmol/L (0.8-1.4), Plasma parathyroid hormone 6.5 pmol/L (0.9-5.4). Which of the following is directly responsible for the increase in intestinal calcium absorption?