This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fetal Skull and Maternal Pelvis, Subject name: Gynaecology & Obstetrics
[{'content': "Conjugate of the diagonal is 'a' cm, obstetric conjugate will be: \nSelect and explain. \nA. a+1 cm, B. a+2 cm, C. a-1 cm, D. a-2 cm", 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is d, i.e. (a -2) cmRef. Williams Obs. 24/e, p 32Obstetric conjugate cannot be measured clinically but can be diagnosed by substracting 1.5-2 cm from diagonal conjugate. Hence if diagonal conjugate is 'a' cm obstetric conjugate will be (a -2) cmObstetric conjugate normally measures 10 cm or more Q.The pelvic inlet is considered to be contracted, if obstetric conjugate is less than 10 cm. Q", 'role': 'assistant'}]