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 is the shoest pelvic diameter? 
Select and explain. 
A. Diagonal conjugate, B. Obstetric conjugate, C. True conjugate, D. All are equal
B: Obstetric conjugate
Answer: B: Obstetric conjugate 
Explanation: Obstetric conjugate is the shoest diameter of the pelvic inlet. It measures the distance between the promontory of the sacrum and the symphysis pubis. Normally, it measures 10 cm or more. This cannot be measured directly by examining, it is calculated by subtracting 1.5 to 2 cm from the diagonal conjugate diameter. Diagonal conjugate is determined by measuring the distance from the lower margin of the symphysis to the sacral promontory. The four diameters of the pelvic inlet are: True conjugate diameter Transverse diameter Oblique conjugate diameter Diagonal conjugate 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. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.