This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: nutrition and digestion, Subject name: Biochemistry
Magnesium 
Select and explain. 
A. Is essential for efficient secretion and peripheral action of PTH, B. Is essential for efficient secretion and peripheral action of calcitonin, C. Is cofactor for vitamin D in bone, D. Competitively occupies calcium slots in bone
A: Is essential for efficient secretion and peripheral action of PTH
Answer: A: Is essential for efficient secretion and peripheral action of PTH 
Explanation: It is uncommon and always due to excessive intake either orally (antacids), rectally (enema) or parenterally. Causes of hypermagnesemia are listed in Box 35.19. Magnesium intoxication causes depression of neuromuscular system, causing lethargy, hypotension, respiratory depression, brady- cardia and weak tendon reflexes. In severe conditions, acute rhabdomyolysis results. Hypermagnesemia induces decrease in serum calcium by inhibiting PTH secretion, which in turn will have deleterious effects.Ref: DM Vasudevan, page no: 412