{"id":5566,"date":"2025-03-20T00:02:00","date_gmt":"2025-03-20T08:02:00","guid":{"rendered":"https:\/\/www.osmosis.org\/blog\/?p=5566"},"modified":"2025-10-01T11:38:24","modified_gmt":"2025-10-01T19:38:24","slug":"usmle-step-1-question-of-the-day-left-sided-weakness","status":"publish","type":"post","link":"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-left-sided-weakness","title":{"rendered":"USMLE\u00ae Step 1 Question of the Day:\u00a0Left-sided weakness"},"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-left-sided-weakness\/#The_correct_answer_to_todays_USMLE%C2%AE_Step_1_Question_is%E2%80%A6\" >The correct answer to today\u2019s USMLE\u00ae Step 1 Question is\u2026<\/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-left-sided-weakness\/#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-left-sided-weakness\/#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-left-sided-weakness\/#Major_Takeaway\" >Major Takeaway&nbsp;<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-left-sided-weakness\/#Want_to_learn_more_about_this_topic\" >Want to learn more about this topic?<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-left-sided-weakness\/#References\" >References&nbsp;<\/a><\/li><\/ul><\/nav><\/div>\n\n<p class=\"wp-block-paragraph\"><strong><em>Each week, Osmosis shares a USMLE\u00ae Step 1-style practice question to test your medical knowledge. Today\u2019s case involves a 66-year-old woman who suddenly develops left-sided weakness. She has a history of hypertension and hyperlipidemia. Neurologic examination reveals right-sided gaze deviation and weakness in her left limbs. Can you identify the most common origin of the thrombus related to her condition?<\/em><\/strong> <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A 66-year-old woman presents to the emergency department after developing acute onset, left-sided weakness. The patient\u2019s daughter reports she found her mother sitting in a chair, unable to lift her left arm or leg. The patient\u2019s past medical history includes hypertension and hyperlipidemia, for which she takes hydrochlorothiazide and atorvastatin. The patient\u2019s temperature is 37\u00b0C (98.6\u00b0F), pulse is 110\/min, respirations are 18\/min, blood pressure is 177\/86 mmHg, and O<sub>2<\/sub> saturation is 97% on room air. Neurologic examination demonstrates a right-sided gaze deviation and 2\/5 strength in the left upper and left lower extremity. An initial electrocardiogram is shown below. <\/p>\n\n\n\n<figure class=\"wp-block-image aligncenter size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"975\" height=\"474\" src=\"https:\/\/hm-healthdigital-multisite.go-vip.net\/osmosisblog\/wp-content\/uploads\/sites\/2\/2025\/02\/image_5864da.png\" alt=\"\" class=\"wp-image-5572\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/02\/image_5864da.png 975w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/02\/image_5864da.png?resize=300,146 300w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/02\/image_5864da.png?resize=768,373 768w\" sizes=\"auto, (max-width: 975px) 100vw, 975px\" \/><figcaption class=\"wp-element-caption\">Image source: <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:RapidAFib150.jpg\" target=\"_blank\" rel=\"noreferrer noopener\">Wikimedia Commons<\/a>.&nbsp;<\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Which of the following structures of the heart is the <strong>most common origin<\/strong> of the thrombus implicated in this patient\u2019s clinical condition?\u00a0<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A. Left atrial appendage<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>B. Right ventricle<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C. Left ventricle<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D. Mitral valve<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E. Tricuspid valve<\/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\u2019s USMLE\u00ae Step 1 Question is\u2026<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A. Left atrial appendage<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Before we get to the Main Explanation, let\u2019s look at the incorrect answer explanations. Skip to the bottom if you want to learn more about 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>B. Right ventricle<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The right ventricle is not the most common site of thrombus formation resulting in ischemic stroke. Thrombus formation and subsequent ischemic stroke do not commonly occur from the right heart unless a patent foramen ovale is present.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C. Left ventricle<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Left ventricular aneurysms following myocardial infarction can serve as the site of thrombus formation and subsequent ischemic stroke; however, it is not the most common site of thrombus formation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D. Mitral valve<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The left atrial appendage, not the mitral valve, is the most common site of thrombus formation and subsequent ischemic stroke from atrial fibrillation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E. Tricuspid valve<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The tricuspid valve is not the most common site of thrombus formation resulting in ischemic stroke. Thrombus formation and subsequent ischemic stroke do not commonly occur from the right heart unless a patent foramen ovale is present.<\/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 presents with evidence of an ischemic stroke\u2013a major complication of untreated atrial fibrillation, which is depicted on this patient\u2019s electrocardiogram.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Atrial fibrillation is the most common sustained arrhythmia in clinical practice and results from unpatterned depolarization of atrial tissues caused by multiple micro-reentry circuits. These reentry circuits can be caused by progressive fibrosis and atrial dilation. Electrocardiographically, atrial fibrillation is characterized by irregular QRS complexes with absent P-waves.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Diastolic filling and subsequent cardiac output may decrease when rapid ventricular response occurs with rates &gt;100 beats per minute. These changes can result in chest pain, shortness of breath, dizziness, and lightheadedness. In addition, hemostasis in the atria can result in thrombus formation, most commonly at the left atrial appendage, owing to its shape and internal trabeculations.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Major_Takeaway\"><\/span>Major Takeaway&nbsp;<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Ischemic stroke is a major complication of untreated atrial fibrillation. Thrombus formation secondary to atrial fibrillation most commonly occurs in the left atrial appendage. <\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Want_to_learn_more_about_this_topic\"><\/span>Want to learn more about this topic?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Watch the Osmosis video:\u00a0<a href=\"https:\/\/www.osmosis.org\/learn\/Ischemic_stroke\">Ischemic Stroke\u00a0video<\/a><\/strong><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"References\"><\/span>References&nbsp;<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Al-Saady NM, Obel OA, Camm AJ. Left atrial appendage: structure, function, and role in thromboembolism. Heart. 1999 Nov;82(5):547-54. doi: 10.1136\/hrt.82.5.547. PMID: 10525506; PMCID: PMC1760793.<\/li>\n<\/ul>\n\n\n\n<figure class=\"wp-block-image aligncenter size-full\"><a href=\"https:\/\/www.osmosis.org\/plans\/md\"><img loading=\"lazy\" decoding=\"async\" width=\"700\" height=\"250\" src=\"https:\/\/hm-healthdigital-multisite.go-vip.net\/osmosisblog\/wp-content\/uploads\/sites\/2\/2025\/02\/health-sci-mo-ad.png\" alt=\"\" class=\"wp-image-5573\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/02\/health-sci-mo-ad.png 700w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/02\/health-sci-mo-ad.png?resize=300,107 300w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/a><\/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&nbsp;<em><strong><a href=\"https:\/\/www.osmosis.org\/login?type=create\" target=\"_blank\" rel=\"noreferrer noopener\">free trial<\/a><\/strong><\/em>&nbsp;and discover why millions of current and future clinicians and caregivers love learning with us.<\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Ace the USMLE\u00ae Step 1 exam by studying with us! Let&#8217;s explore a clinical case involving a 66-year-old woman with left-sided weakness. Learn to identify the most common origin of thrombus in patients experiencing ischemic strokes, particularly in the context of atrial fibrillation. Test your clinical knowledge and improve your diagnostic skills with this engaging practice question!<\/p>\n","protected":false},"author":202,"featured_media":5575,"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":[303,150,452,84,219,245,279,454,453,185],"class_list":["post-5566","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","tag-atrial-fibrillation","tag-clinical-practice","tag-ischemic-stroke","tag-medical-education","tag-neurology","tag-nursing-education","tag-patient-assessment","tag-stroke-management","tag-thrombus","tag-usmle"],"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:\u00a0Left-sided weakness - 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-left-sided-weakness\" \/>\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:\u00a0Left-sided weakness - Osmosis Blog\" \/>\n<meta property=\"og:description\" content=\"Ace the USMLE\u00ae Step 1 exam by studying with us! Let&#039;s explore a clinical case involving a 66-year-old woman with left-sided weakness. Learn to identify the most common origin of thrombus in patients experiencing ischemic strokes, particularly in the context of atrial fibrillation. 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