This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Discoloration of tooth in tetracycline therapy is because of formation of: 
Select and explain. 
A. Calcium orthophosphate, B. Dicalcium phosphate dihydrate, C. Calcium oxide, D. Tetracalcium phosphate
A: Calcium orthophosphate
Answer: A: Calcium orthophosphate 
Explanation: Tetracycline and minocycline: Unsightly discoloration of both dentitions results from excessive intake of tetracycline and minocycline during the development of teeth. Chelation of tetracycline molecule with calcium in hydroxyapatite crystals forms tetracycline orthophosphate which is responsible for discolored teeth.