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: Pharmacology
[{'content': 'Which of the following drug does not cross the placental barrier? \nSelect and explain. \nA. Heparin, B. Morphine, C. Phenytoin, D. Diazepam', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Heparin does not cross the placental barrier due to its high molecular weight and its negative charge. So it is the anticoagulant of choice during pregnancy. Ref: Drugs During Pregnancy and Lactation By Christof Schaefer, Page 86 ; Essentials of Medical Pharmacology By K D Tripathi, 5th Edition, Pages 561-3', 'role': 'assistant'}]