{"id":3703,"date":"2024-05-08T18:17:00","date_gmt":"2024-05-08T18:17:00","guid":{"rendered":"https:\/\/www.osmosis.org\/blog\/?p=3703"},"modified":"2025-10-01T11:39:34","modified_gmt":"2025-10-01T19:39:34","slug":"usmle-step-1-question-of-the-day-sunlight-sensitivity","status":"publish","type":"post","link":"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity","title":{"rendered":"USMLE\u00ae Step 1 Question of the Day: Sunlight sensitivity"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_80 ez-toc-wrap-center counter-hierarchy ez-toc-counter ez-toc-custom ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">In This Article<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\/#The_correct_answer_to_todays_USMLE%C2%AE_Step_1_Question_is%E2%80%A6\" >The correct answer to today&#8217;s USMLE\u00ae Step 1 Question is&#8230;<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\/#Incorrect_Answer_Explanations\" >Incorrect Answer Explanations<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\/#Main_Explanation\" >Main Explanation<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\/#Major_Takeaway\" >Major Takeaway<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\/#References\" >References<\/a><\/li><\/ul><\/nav><\/div>\n\n<p class=\"wp-block-paragraph\"><strong><em>Prepare for USMLE Step 1 with this case! A 4-year-old boy with increased sensitivity to sunlight and facial skin changes since birth is brought in for evaluation. What&#8217;s the most likely diagnosis?<\/em><\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A 4-year-old boy is brought to the office by his parent because of increased sensitivity to sunlight. His parent reports that the patient sunburns easily since birth, even with minimal sun exposure. He has been meeting all developmental milestones and just recently started riding his tricycle with no assistance. Family history is significant for cutaneous squamous cell carcinoma in his paternal grandfather. \u00a0Vitals are within normal limits for age. Physical examination shows bilateral conjunctival injection and dryness of the facial skin with rough texture and erythematous macules and freckles. A complete neurological examination shows no abnormalities. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Which of the following processes of DNA repair is <em>most likely<\/em> to be defective in this patient?\u00a0<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A.&nbsp;DNA mismatch repair&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>B.&nbsp;Nonhomologous end joining repair&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C.&nbsp;Homologous recombination&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D.&nbsp;Nucleotide excision repair<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E.&nbsp;Base excision repair&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"has-text-align-center wp-block-paragraph\"><em>Scroll down for the correct answer!<\/em><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"The_correct_answer_to_todays_USMLE%C2%AE_Step_1_Question_is%E2%80%A6\"><\/span>The correct answer to today&#8217;s USMLE\u00ae Step 1 Question is&#8230;<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D.\u00a0Nucleotide excision repair<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Before we get to the Main Explanation, let&#8217;s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Incorrect_Answer_Explanations\"><\/span>Incorrect Answer Explanations<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A.\u00a0DNA mismatch repair<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong>&nbsp;Defective DNA mismatch repair results in Lynch syndrome (hereditary nonpolyposis colorectal cancer), which is an autosomal dominant genetic disorder that is strongly associated with colorectal cancer. Most patients are asymptomatic, but gastrointestinal bleeding and abdominal pain are the usual presentation. This patient\u2019s history of photosensitivity makes this disorder unlikely.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>B.\u00a0Nonhomologous end joining repair<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong>&nbsp;Nonhomologous end joining refers to joining the ends of DNA fragments to repair the double-stranded breaks. Failure of this repair mechanism is usually observed in ataxia-telangiectasia, due to an abnormal&nbsp;ATM&nbsp;gene. The condition usually manifests in children as cerebellar ataxia, spider angiomas and recurrent infections. Dry skin and photosensitivity is atypical.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C.\u00a0Homologous recombination\u00a0<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong>&nbsp;Homologous recombination&nbsp;refers to repair of damaged DNA using a complementary DNA strand from intact homologous double-stranded DNA. Failure of this mechanism results in Fanconi anemia, which presents with short stature, caf\u00e9-au-lait spots, and thumb\/radial defects. Dry pigmented skin with increased sensitivity to sunlight is not seen.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E.\u00a0Base excision repair\u00a0<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong>&nbsp;Base excision repair plays an important role in the repair of spontaneous or toxic deamination (removal of amino groups from nitrogen bases) that results from chemicals like nitrites or nitrosamine. A defect in base excision repair can increase the risk of developing colorectal cancer but is not associated with xeroderma pigmentosum.&nbsp;&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Main_Explanation\"><\/span>Main Explanation<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This patient\u2019s presentation with&nbsp;photosensitive skin&nbsp;since infancy, in combination with physical examination findings of&nbsp;dry and erythematous facial skin with freckles&nbsp;and&nbsp;bilateral conjunctivitis,&nbsp;is strongly suggestive of&nbsp;xeroderma pigmentosum&nbsp;(XP).&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Normally, harmful ultraviolet (UV) radiation creates&nbsp;pyrimidine dimers&nbsp;commonly between two adjacent thymines on one of the DNA strands, distorting the DNA strand in that region of the molecule.&nbsp;Nucleotide excision repair&nbsp;fixes this damage in a stepwise manner as follows:&nbsp;<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li>Initially,&nbsp;endonucleases&nbsp;make two incisions in the DNA strand on the 3\u2019 side and 5\u2019 side of the damaged bases, leaving a fragment of 12 to 24 nucleotides with free ends, exposing them to exonucleases.&nbsp;<\/li>\n\n\n\n<li>Exonucleases&nbsp;remove these damaged nucleotides.&nbsp;<\/li>\n\n\n\n<li>DNA polymerase&nbsp;inserts new nucleotides, and&nbsp;<\/li>\n\n\n\n<li>DNA ligase&nbsp;seals the bonds once again&nbsp;<\/li>\n<\/ol>\n\n\n\n<figure class=\"wp-block-image aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" height=\"1024\" width=\"428\" src=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/nucleotide-excision-repair.png?w=428\" alt=\"NUCLEOTIDE EXCISION REPAIR\nA gene sequence showing a damaged base --&gt;1. Denaturation bubble --&gt; 2. Nicking (endonuclease) --&gt; 3. Digestion (exonuclease) --&gt; 4. Resynthesis (DNA polymerase) --&gt; 5. Ligation (DNA Ligase)\" class=\"wp-image-3706\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/nucleotide-excision-repair.png 486w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/nucleotide-excision-repair.png?resize=125,300 125w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/nucleotide-excision-repair.png?resize=428,1024 428w\" sizes=\"auto, (max-width: 428px) 100vw, 428px\" \/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">XP is a rare&nbsp;autosomal recessive&nbsp;disorder that results from mutations in the genes that are involved in nucleotide excision repair. It primarily manifests in childhood with pigmented skin changes (e.g.&nbsp;freckling), skin atrophy, dryness, telangiectasia, and ocular damage which can present as ectropion,&nbsp;conjunctival injection, and\/or corneal scarring.&nbsp;Progressive neurodegeneration&nbsp;is also seen in some cases, and this complication may present with sensorineural hearing loss, intellectual impairment, speech delays, ataxia, and peripheral neuropathy.&nbsp;Strict sun avoidance, protection, and frequent skin examination are appropriate management options.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Major_Takeaway\"><\/span>Major Takeaway<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Xeroderma pigmentosum&nbsp;is an&nbsp;autosomal recessive&nbsp;genetic disorder that results from&nbsp;defective&nbsp;nucleotide excision repair mechanisms. It usually manifests in childhood as pigmentary skin changes, signs of photoaging, and ocular damage.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"References\"><\/span>References<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Farrington, S. M., Tenesa, A., Barnetson, R., Wiltshire, A., Prendergast, J., Porteous, M., &#8230; &amp; Dunlop, M. G. (2005). Germline susceptibility to colorectal cancer due to base-excision repair gene defects. The American Journal of Human Genetics, 77(1), 112-119.&nbsp;<\/li>\n\n\n\n<li>Lee, K. J., Piett, C. G., Andrews, J. F., Mann, E., Nagel, Z. D., &amp; Gassman, N. R. (2019). Defective base excision repair in the response to DNA damaging agents in triple negative breast cancer. PloS one, 14(10).\u00a0<\/li>\n<\/ul>\n\n\n\n<figure class=\"wp-block-image size-large\"><a href=\"https:\/\/www.osmosis.org\/ultimate-guide\/usmle-step-1\" target=\"_blank\" rel=\"noreferrer noopener\"><img loading=\"lazy\" decoding=\"async\" width=\"700\" height=\"250\" src=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/the-ultimate-guide-usmle-step-1-banner.png?w=700\" alt=\"The Ultimate Guide to the USMLE Step 1: Everything you need to know\" class=\"wp-image-760\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/the-ultimate-guide-usmle-step-1-banner.png 700w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/the-ultimate-guide-usmle-step-1-banner.png?resize=300,107 300w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/a><figcaption class=\"wp-element-caption\"><em><em><sub>The United States Medical Licensing Examination (USMLE\u00ae) is a joint program of the Federation of State Medical Boards (FSMB\u00ae) and National Board of Medical Examiners (NBME\u00ae). Osmosis is not affiliated with NBME nor FSMB.&nbsp;<\/sub><\/em><\/em><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><em><strong>Want more USMLE\u00ae Step 1 practice questions? Try Osmosis by Elsevier today! Access your\u00a0<em><strong><a href=\"https:\/\/www.osmosis.org\/login?type=create\" target=\"_blank\" rel=\"noreferrer noopener\">free trial<\/a><\/strong><\/em>\u00a0and find out why millions of current and future clinicians and caregivers love learning with us.<\/strong><\/em><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Prepare for USMLE Step 1 with this case! A 4-year-old boy with increased sensitivity to sunlight and facial skin changes since birth is brought in for evaluation. What&#8217;s the most likely diagnosis? A 4-year-old boy is brought to the office by his parent because of increased sensitivity to sunlight. His parent reports that the patient [&hellip;]<\/p>\n","protected":false},"author":202,"featured_media":3705,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[27,20,16,1366,1369,44],"tags":[],"class_list":["post-3703","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-medicine","category-exam-prep","category-do","category-questions","category-step-1-questions","category-step-1"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v23.6 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>USMLE\u00ae Step 1 Question of the Day: Sunlight sensitivity - Osmosis Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"USMLE\u00ae Step 1 Question of the Day: Sunlight sensitivity - Osmosis Blog\" \/>\n<meta property=\"og:description\" content=\"Prepare for USMLE Step 1 with this case! A 4-year-old boy with increased sensitivity to sunlight and facial skin changes since birth is brought in for evaluation. What&#8217;s the most likely diagnosis? A 4-year-old boy is brought to the office by his parent because of increased sensitivity to sunlight. His parent reports that the patient [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\" \/>\n<meta property=\"og:site_name\" content=\"Osmosis Blog\" \/>\n<meta property=\"article:published_time\" content=\"2024-05-08T18:17:00+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2025-10-01T19:39:34+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/09\/usmle-sunlight-sensitivity.webp\" \/>\n\t<meta property=\"og:image:width\" content=\"1080\" \/>\n\t<meta property=\"og:image:height\" content=\"1080\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/webp\" \/>\n<meta name=\"author\" content=\"Marina Horiates Kerekes, MD &amp; Team\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":[\"Article\",\"BlogPosting\"],\"@id\":\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-sunlight-sensitivity\"},\"author\":{\"name\":\"Marina Horiates Kerekes, MD &amp; 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