Roxanita
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Topics: 16
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Q-1) An 8-year-old boy is brought to a dermatology clinic by his parents. They note that his skin is extremely sensitive to sunlight and his chart indicates that he has had multiple melanomas removed over his lifetime. This patient likely has
(A)a defect in AP endonuclease
(B)a defect in excision repair of ultraviolet-damaged DNA
(C)been exposed to pyrimidine analogs
(D)defective proofreading
(E)deletion of his helicase gene
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| rida
Forum Guru
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B should be it.
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| Idiopathic
Forum Guru
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lookslike xeroderma pigmentosum to me...defect in repair of UVB-damaged DNA.
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| Roxanita
Forum Senior
Topics: 16
Posts: 108
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Very good, The correct answer is B 
This patient has Xeroderma Pigmentosum ...Let's read something about this :wink:... Autosomal recessive trait; this disease is characterized by an extreme sensitivity to sunlight, skin changes, and a predisposition to malignancy. The disease results from a defective excision repair mechanism (Excision Endonuclease Enzyme) for UV-damaged DNA.
About the other answers:
choice a) A defect in the AP Endonuclease (choice A) leads to ataxia-telangiectasia, characterized by hypersensitivity to ionizing radiation, dilated blood vessels, and lymphomas.
choice c) Pyrimidine analogs; such as 5-fluorodeoxyuridine, inhibit thymidylate synthase, an enzyme critical for DNA synthesis. Insertion of these analogs stops elongation of the DNA chain; therefore, they are useful as chemotherapeutic agents.
choice D) Defective proofreading; would affect DNA replication in all body tissues, not just those exposed to UV.
Choice E) Helicases are enzymes responsible for unwinding the DNA before DNA replication can begin. Helicase abnormalities would affect DNA replication in all body tissues, not just those exposed to UV.
___________________
"We all basically have 2 choices in life: Let things stay the way they are or work to make them better"
-Julia Havey
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