This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Physiology
Which of the following cause low serum calcium? 
Select and explain. 
A. Vit D deficiency, B. Parathyroid, C. GH, D. Glucocoicoids
A: Vit D deficiency
Answer: A: Vit D deficiency 
Explanation: Vitamin D is a group of fat-soluble secosteroids responsible for enhancing intestinal absorption of calcium and phosphate. Calcitriol circulates as a hormone in the blood, regulating the concentration of calcium and phosphate in the bloodstream and promoting the healthy growth and remodeling of boneRef: Ganong&;s review of medical physiology; 24th edition; page no:-379