This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal tract, Subject name: Pharmacology
Which of the following drugs crosses the blood-brain barrier 
Select and explain. 
A. Loperamide, B. Kaolin, C. Methylcellulose, D. Diphenoxylate
D: Diphenoxylate
Answer: D: Diphenoxylate 
Explanation: Diphenoxylate It is a synthetic opioid, chemically related to pethidine; used exclusively as constipating agent; action is similar to codeine. The antidiarrhoeal action is most prominent, but because it is absorbed systemically and crosses blood-brain barrier--CNS effects do occur. Atropine is added in subpharmacological dose to discourage abuse by taking several tablets. adverse effects:- respiratory depression, paralytic ileus and toxic megacolon in children. contraindication:- contraindicated below 6 years of age. Ref:- kd tripathi; pg num:-686