611. Which of the following tissues begins formation first:
A. Enamel.
B. Dentin.
C. Pulp.
D. Cementum.

612. Attrition is characterized by:
A. Being a condition of normal wear of the occlusal and proximal surfaces.
B. A chemical loss of occlusal tooth structure, but not from dental caries.
C. A loss of crown structure on the occlusal surface by mechanical means.
D. None of the above.

613. In mature dental tissues, the only tissue whose cells are lost is:
A. Dentin.
B. Enamel.
C. Cementum.
D. Pulp.

614. Enamel is by weight:
A. 88-90 % mineral, 10-12 % organic matrix and water.
B. 76% mineral, 20% organic matrix, 4 water.
C. 25% mineral, 12% organic matrix, 3% water.
D. 96% mineral, 1-2% organic material, 2-3% water.

615. Which of the following is not true about Enamel rod ?
A. They run from the dentinoenamel junction to the crown surface.
B. Length of all enamel rods is equal.
C. They are the basic structural units of E.
D. They may be hexagonal ,rounded, oval or keyhole shape in cross section .

616. Which of the following statements about E .is false?
A. The heads of enamel rods are directed cervically, the tails occlusally.
B. Rod sheaths are relatively hypocalcified areas surrounding the rods.
C. Human E rods in cross section have an outline similar to a keyhole.
D. Hydroxyapatite crystals run parallel to the long axis of the rods and perpendicular to it in the interrodregion .

617. It is particularly difficult to finish the wall of a cavity preparation in cuspal enamel because of the presence in that area of:
A. Tufts.
B. Horizontal E. rods.
C. Gnarled E.
D. Spindles.

618. Enamel tufts and Enamel lamella resemble each other, in that both:
A. Are formed after to tooth eruption.
B. Extend over full thickness of the enamel.
C. Are hypermineralized structures.
D. Are oriented in a vertical direction perpendicular on the DEJ.

619. All of the following statements about the incremental lines of Retzius in Enamel are true except:
A. In longitudinal section they run obliquely, relative to D.E.J.
B. Perikymata in newly erupted teeth are manifestations of the lines of Retzius.
C. They are formed as a result of rhythmic variations in the calcification.
D. Under no conditions can a line of Retzius to be a neonatal line.

620. E. has no possibility of self repair because:
A. It has a small percentage of organic content.
B. Its formative cells are lost once it is fully formed and the tooth erupted.
C. It is essentially a keratin tissue and has no blood vessels.
D. It has no direct connection with the active cells of the pulp tissue.

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