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
Which of the following drug does not cross the placental barrier? 
Select and explain. 
A. Heparin, B. Morphine, C. Phenytoin, D. Diazepam
A: Heparin
Answer: A: Heparin 
Explanation: 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