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: Gynaecology & Obstetrics
Which of the following pelvic diameter measures the distance from the lower margin of the symphysis to the sacral promontory? 
Select and explain. 
A. Transverse diameter, B. Diagonal conjugate, C. Obstetric conjugate, D. True conjugate diameter
B: Diagonal conjugate
Answer: B: Diagonal conjugate 
Explanation: Diagonal conjugate diameter is determined by measuring the distance from the lower margin of the symphysis to the sacral promontory. Obstetric conjugate is the shoest distance between the promontory of the sacrum and the symphysis pubis. It is usually calculated by subtracting 1.5 to 2 cm from the diagonal conjugate. Normally, this measures 10 cm or more. The transverse diameter is at right angles to the obstetrical conjugate and it represents the greatest distance between the linea terminalis on either side. It is usually 13cm in diameter. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 2. Maternal Anatomy.