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: Anaesthesia
Which of the following best explains why Lignocaine has a higher fetal-to-maternal plasma ratio when compared with bupivacaine? 
Select and explain. 
A. Bupivacaine has low molecular weight, B. Lignocaine has higher protein-binding, C. Bupivacaine has a lower dissociation constant (pKa), D. Lignocaine is less protein bound
D: Lignocaine is less protein bound
Answer: D: Lignocaine is less protein bound 
Explanation: Lignocaine is  less protein bound than bupivacaine. Unbound form of drug thus easily gets transferred across placenta.