561.Striae of Retzius:
A. Result from enamel crack.
B. Separate prenatal from postnatal enamel.
C. Are due to rhythmic apposition of enamel.
D. Are hypermineralized structure.

562. Enamel Rods:
A. Their number varies in different teeth.
B. Have an average width of 1um.
C. Are Present at the enamel surface.
D. Have a straight course at the cusps of teeth.

563. Enamel is formed of:
A. 76% minerals & 24% organic materials and water.
B. 96% minerals & 4% organic materials and water.
C. 86% minerals & 14% organic materials and water.
D. 99% minerals & 1% organic materials and water.

564.which of the following is absent in all premolars:
A. Enamel spindle.
B. Enamel tuft.
C. Neonatal line.
D. Enamel lamella.

565. Cross striations in enamel:
A. Are an optical phenomena.
B. Result from abrupt change in environment before and after birth.
C. Result from mineralization disorder.
D. Appear to be periodic bands at 4 microns interval across the rod.

566. The prism sheath:
A. Is a dense continuous sheath separating rod from inter-rod.
B. Result from abrupt change in crystals orientation.
C. Is more mineralized than rod and inter-rod.
D. Found in prismless enamel.

567. The complex arrangement of rods over the cusps of teeth is termed:
A.Prismless enamel.
B. Gnarled enamel.
C. Prenatal enamel.
D. Postnatal enamel.

568. The origin of enamel is:
A. neural crest cells
B. Ectoderm.
C. Mesoderm.
D.Ectomesenchyme.

569.Hypocalcified areas in enamel are:
A. Enamel spindles.
B.Dentino-enamel junction.
C.Interprismatic substances.
D. Enamel tufts.

570.Prismless enamel:
A. Is less mineralized than prismatic enamel.
B. Is found only in the cervical area of permanent teeth.
C. Is harder than prismatic enamel.
D. Found near the dentino-enamel junction.

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