What anatomy is labeled as letter A in the image below? 
Definitions:
Enamel
The hard, protective outer layer of the tooth, made primarily of minerals and the hardest material in the human body.
Cementum
A specialized calcified substance covering the root of a tooth.
Crown
The part of a tooth that is visible above the gum line, or the top portion of a dental implant that replaces the natural tooth.
Mesenteries
Folds of peritoneum that attach the intestines to the posterior abdominal wall, providing support and housing blood vessels, nerves, and lymphatic vessels.
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