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: Pathology
Delicate, brown or black pigmented non cariogenic plaque found on the enamel at the cervical margin of the tooth is: 
Select and explain. 
A. Burtonian line, B. Argyria, C. Bismuth line, D. Mesenteric line
D: Mesenteric line
Answer: D: Mesenteric line 
Explanation: A delicate pigmented dental plaque, called the mesenteric line by some, was described by Pickerill to be a plaque of brown or black dots that coalesce to form a thin, dark line on the enamel at the cervical margin of the tooth. Pickerill, F Bibby and Shourie noted that the presence of this line is often associated with a relative freedom from dental caries.
Ref: Shafer's textbook of oral pathology 7th edition page 386