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NBME 23 Answers

nbme23/Block 1/Question#39

A 65-year-old man who is a farmer comes to the ...

Disruption of DNA strands secondary to ionization effect

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 +9  upvote downvote
submitted by sajaqua1(347),

First, critically, it is not described as ion radiation (alpha and beta particles) but as radiation that has an ionization effect. Our farmer has been out standing in his field, soaking up the sunlight. UV-B is wave radiation that induces the formation of free radicals. These free radicals in turn do damage to the cell, including the formation of pyrimidine dimers. This DNA damage is the bases for his squamous cell carcinoma. For more on UV-B induced free radical damage of DNA, https://www.ncbi.nlm.nih.gov/pubmed/16765564 . For more information on UV radiation from the American Cancer Society, please visit https://www.cancer.org/cancer/cancer-causes/radiation-exposure/uv-radiation/uv-radiation-what-is-uv.html

A) Decreased function of the skin as a barrier to carcinogens- the patient has no history suggest exposure to carcinogens besides UV light, and as long as the skin is intact and relatively healthy it should function as some kind of barrier. C) Increased conversion of 25-D3 to 1,25-D3 - this conversion takes place in the kidneys via 1-alpha-hydroxylase, also expressed in granulomas (and so 1,25-D3 is often elevated in sarcoidosis). This would not explain cancer. D) Increased mitotic activity through stimulation of melanocytes- our patient is diagnosed with squamous cell carcinoma, not melanoma. E) Increased susceptibility to skin trauma- while the damage from UV light to collagen does make skin thinner and more readily damaged, this would not explain squamous cell carcinoma.

 +5  upvote downvote
submitted by keycompany(182),

Can somebody please explain how nonionizing radiation has an ionizing effect.

uslme123  "technically non-ionizing, can produce photochemical reactions that are damaging to molecules by means other than simple heat. Since these reactions are often very similar to those caused by ionizing radiation, often the entire UV spectrum is considered to be equivalent to ionization radiation in its interaction with many systems (including biological systems)." -- https://en.wikipedia.org/wiki/Non-ionizing_radiation#Near_ultraviolet_radiation I'm guessing NBME reads wiki lmao? +2  

 +3  upvote downvote
submitted by keycompany(182),

Figure it out after reading a review. UVA/UVB under normal circumstances is considered nonionizing. However, the only difference between nonionizing and ionizing radiation is the strength of the radiation rays. Because radiation does not dissipate from the body, prolonged UVB exposure can cause radiation to build up in the skin and cause an ionization effect. This is the same reason that repetitive X-Rays and CT Scans can increase your risk for cancer, even years down the line.