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: ENT
All of the following form the boundary of MacEwen's triangle except - 
Select and explain. 
A. Temporal line, B. Posterosuperior segment of bony external auditory canal, C. Promontory, D. Tangent drawn to the external auditory meatus
C: Promontory
Answer: C: Promontory 
Explanation: Mastoid antrum is marked externally on the surface by suprameatal (Mac Ewen’s) triangle.