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
[{'content': "Conjugate of the diagonal is taken as 'a' cm, then 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: Obstetrical conjugate\n\nIt is the shortest distance between the sacral promontory and the symphysis pubis\nNormally, this measures 10 cm or more, but it cannot be measured directly with examining fingers\nThus, for clinical purposes, the obstetrical conjugate is estimated indirectly by subtracting 1.5 to 2 cm from the diagonal conjugate, which is determined by measuring the distance from the lowest\xa0\xa0margin of the symphysis to the sacral promontory', 'role': 'assistant'}]