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 cephalometric point represents the centre of ramus of mandible? 
Select and explain. 
A. Xi-point, B. Pm-point, C. Ptm-point, D. N-point
A: Xi-point
Answer: A: Xi-point 
Explanation: Xi point:
A point located at the geometric centre of the ramus. The location of the Xi point can be found geometrically by drawing a line through the porion–orbitale (FH) and perpendicularly through the PT vertical (PTV).