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
Which of the following is the single most important predictor of clinical success of a cast post and core? 
Select and explain. 
A. Amount of remaining coronal tooth structure., B. Post length., C. Post diameter., D. Positive horizontal stop.
A: Amount of remaining coronal tooth structure.
Answer: A: Amount of remaining coronal tooth structure. 
Explanation: The length, canal enlargement, and a finish line for the post are unimportant, if there is no sound remaining coronal tooth structure to get a ferrule of the final restoration.