{"id":8924,"date":"2025-12-24T00:01:53","date_gmt":"2025-12-24T08:01:53","guid":{"rendered":"https:\/\/www.osmosis.org\/blog\/?p=8924"},"modified":"2026-04-14T11:46:17","modified_gmt":"2026-04-14T19:46:17","slug":"usmle-step-1-question-of-the-day-class-i-antiarrhythmics","status":"publish","type":"post","link":"https:\/\/www.osmosis.org\/blog\/usmle-step-1-question-of-the-day-class-i-antiarrhythmics","title":{"rendered":"USMLE\u00ae Step 1 Question of the Day:\u00a0Class I antiarrhythmics"},"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-class-i-antiarrhythmics\/#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-class-i-antiarrhythmics\/#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-class-i-antiarrhythmics\/#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-class-i-antiarrhythmics\/#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-class-i-antiarrhythmics\/#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-class-i-antiarrhythmics\/#References\" >References<\/a><\/li><\/ul><\/nav><\/div>\n\n<p class=\"wp-block-paragraph\"><em>Today\u2019s USMLE\u00ae Step 1 question of the day features a <strong>57-year-old man post\u2013myocardial infarction<\/strong> who develops <strong>palpitations and lightheadedness<\/strong>. He\u2019s treated with a medication that <strong>decreases the slope of phase 0 depolarization<\/strong>. Can you figure it out?<\/em><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A 57-year-old man is admitted to the <strong>coronary care unit after an acute myocardial infarction<\/strong> and <strong>placement of a drug-eluting stent in the left main coronary artery<\/strong>. While in the post-operative suite, the patient begins experiencing <strong>palpitations and lightheadedness<\/strong>. His rhythm strip is demonstrated below. He is immediately given a <strong>medication that decreases the slope of phase 0 of depolarization<\/strong>. The medication helps <strong>terminate the arrhythmia<\/strong>.<\/p>\n\n\n\n<figure class=\"wp-block-image aligncenter size-full is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"371\" height=\"145\" src=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_e966af.png\" alt=\"\" class=\"wp-image-8940\" style=\"width:392px;height:auto\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_e966af.png 371w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_e966af.png?resize=300,117 300w\" sizes=\"auto, (max-width: 371px) 100vw, 371px\" \/><figcaption class=\"wp-element-caption\">Image reproduced from <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Lead_II_rhythm_ventricular_tachycardia_Vtach_VT_(cropped).JPG\" target=\"_blank\" rel=\"noreferrer noopener\">Wikimedia Commons<\/a><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><em>Which of the following best describes the <strong>mechanism of action<\/strong> of this medication?<\/em><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A. Selective blockade of fast sodium channels<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>B. Decrease in SA and AV nodal activity<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C. Blockade of potassium rectifier currents<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D. Blockade of voltage dependent calcium channels<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E. Increase in potassium efflux via blockade of the AV node<\/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<figure class=\"wp-block-embed is-type-video is-provider-youtube wp-block-embed-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio\"><div class=\"wp-block-embed__wrapper\">\n<iframe loading=\"lazy\" title=\"Osmosis.org at the University College London, UK\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/DIp3u0oTLo4?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n<\/div><\/figure>\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. Selective blockade of fast sodium channels<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Correct:<\/strong> See Main Explanation.<\/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. Decrease in SA and AV nodal activity<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong> <strong>\ua7b5-blockers<\/strong> work via <strong>selective \ua7b5-blockade<\/strong>, <strong>decreasing intracellular cAMP<\/strong>, and subsequently <strong>decreasing Ca2+ current<\/strong>. This mechanism ultimately <strong>decreases the slope of the phase 4 action potential.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>C. Blockade of potassium rectifier currents<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong> <strong>Class III antiarrhythmic medications<\/strong> inhibit <strong>potassium channels responsible for repolarization<\/strong> of the cardiac membrane. These medications <strong>increase action potential duration<\/strong>, <strong>effective refractory period<\/strong>, and <strong>QT interval prolongation.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>D. Blockade of voltage dependent calcium channels<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong> <strong>Class IV antiarrhythmic medications<\/strong> work by <strong>inhibiting voltage-dependent calcium channels<\/strong> and subsequently <strong>decreasing conduction velocity<\/strong>, <strong>increasing the PR interval<\/strong>, and <strong>increasing the effective refractory period.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>E. Increase in potassium efflux via blockade of the AV node<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Incorrect:<\/strong> <strong>Adenosine<\/strong> works via <strong>blockade of A1 adenosine receptors<\/strong>, which <strong>increases potassium efflux<\/strong> and <strong>blocks the AV node<\/strong>. It is <strong>not used to treat ventricular tachycardia.<\/strong><\/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 <strong>ventricular tachycardia<\/strong> after having an <strong>acute myocardial infarction<\/strong>. He is given a <strong>medication that decreases the slope of phase 0 of depolarization<\/strong>, which leads to <strong>termination of the arrhythmia<\/strong>. This mechanism of action is consistent with that of <strong>class I antiarrhythmics<\/strong>. These medications work primarily via <strong>selective blockade of fast sodium channels<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Class I antiarrhythmics<\/strong> are divided into <strong>three major classes: A, B, and C<\/strong>. However, all of them work via the same mechanism of action. <strong>Sodium channel blockade slows the upstroke of the sodium-dependent action potential (phase 0)<\/strong> and <strong>prolongs the QRS duration<\/strong>. However, each group has a different affinity for the fast sodium channel, resulting in <strong>group 1A prolonging the action potential duration<\/strong>, <strong>1B shortening the actional potential duration<\/strong>, and <strong>1C having no effect on the action potential duration<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">These medications have a predilection to <strong>slow conduction in ischemic and depolarized cells<\/strong>. For this reason, <strong>class I antiarrhythmics<\/strong> are said to exhibit <strong>\u201cstate-dependent\u201d action<\/strong>, meaning that they <strong>bind to sodium channels when they are open or inactivated<\/strong> and much less readily when they are fully repolarized and resting. This phenomenon means these medications will <strong>selectively depress tissue that is frequently depolarized<\/strong>, making them useful in treating <strong>reentrant tachycardias and ventricular tachycardias<\/strong>.\u00a0<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"975\" height=\"926\" src=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_bb93ee.png\" alt=\"Infographic titled \u201cCharacteristics of Class I Antiarrhythmics.\u201d\n\nIt explains that all Class I antiarrhythmics slow the upstroke of the sodium-dependent action potential (phase 0) and prolong the QRS interval. The chart is divided into three columns: Class IA, Class IB, and Class IC.\n\nClass IA\n\nEffect: Potassium channel inhibition \u2192 prolongs action potential duration.\n\nGraph: Solid line shows normal action potential; dashed red line shows prolonged duration.\n\nExamples: Quinidine, Procainamide, Disopyramide.\n\nClass IB\n\nEffect: Potassium channel activation \u2192 shortens action potential duration.\n\nGraph: Dashed red line shows shorter duration compared to solid baseline.\n\nExamples: Lidocaine, Mexiletine.\n\nClass IC\n\nEffect: No effect on potassium channels \u2192 no change in action potential duration.\n\nGraph: Dashed red line overlaps the solid baseline, showing no duration change.\n\nExamples: Flecainide, Propafenone.\" class=\"wp-image-8939\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_bb93ee.png 975w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_bb93ee.png?resize=300,285 300w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2025\/12\/image_bb93ee.png?resize=768,729 768w\" sizes=\"auto, (max-width: 975px) 100vw, 975px\" \/><\/figure>\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\"><strong>Class I antiarrhythmics block fast sodium channels<\/strong> and <strong>slow the upstroke of the sodium-dependent action potential (phase 0)<\/strong>. These medications exhibit <strong>state-dependent action<\/strong>, meaning they will <strong>selectively depress tissue that is frequently depolarized<\/strong>.\u00a0<\/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 this Osmosis video: <a href=\"https:\/\/www.osmosis.org\/learn\/Class_I_antiarrhythmics:_Sodium_channel_blockers\">Class I antiarrhythmics: Sodium channel blockers<\/a><\/strong><\/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>Arnsdorf, M.F. (199) The cellular basis of cardiac arrhythmias. A matrical perspective. <em>Annals of the New York Academy of Sciences<\/em>. 601, 263-280. Doi: 10.1111\/j.1749-6632.1990.tb37306.x.&nbsp;<\/li>\n\n\n\n<li>Dan, G.A., Martinez-Rubio, A., Agewall, S., et al. (2018) Antiarrhythmic drugs-clinical use and clinical decision making: a consensus document from the European Heart Rhythm Association (EHRA) and European Society of Cardiology (ESC) Working Group on Cardiovascular Pharmacology, endorsed by the Heart Rhythm Society (HRS), Asia-Pacific Heart Rhythm Society (APHRS) and International Society of Cardiovascular Pharmacotherapy (ISCP). <em>Europace<\/em>. 20(5), 731-732. Doi: 10.1093\/europace\/eux373.&nbsp;<\/li>\n\n\n\n<li>Trevor, A., Katzung, B., Knuidering-Hall, M. (2015) Katzung &amp; Trevor&#8217;s pharmacology: Examination &amp; board review (11th ed.). New York, NY: McGraw-Hill. ISBN: 978-0071826358.<\/li>\n<\/ul>\n\n\n\n<figure class=\"wp-block-image size-large\"><a href=\"https:\/\/www.osmosis.org\/plans\"><img loading=\"lazy\" decoding=\"async\" width=\"700\" height=\"250\" src=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/10\/Blog_Display_Ads_MOBILE3_2023.png?w=700\" alt=\"\" class=\"wp-image-4729\" srcset=\"https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/10\/Blog_Display_Ads_MOBILE3_2023.png 700w, https:\/\/www.osmosis.org\/blog\/wp-content\/uploads\/sites\/2\/2024\/10\/Blog_Display_Ads_MOBILE3_2023.png?resize=300,107 300w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/a><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><em>Want more <strong>USMLE\u00ae Step 1<\/strong> practice questions? Try <strong>Osmosis from Elsevier<\/strong> today! Access your&nbsp;<em><a href=\"https:\/\/www.osmosis.org\/plans\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>free trial<\/strong><\/a><\/em>&nbsp;and discover why millions of current and future <strong>clinicians <\/strong>and <strong>caregivers <\/strong>love <strong>learning with us<\/strong>.<\/em><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>A patient with arrhythmia after myocardial infarction receives a medication that alters phase 0 depolarization. Explore how this drug modulates cardiac electrical activity.<\/p>\n","protected":false},"author":204,"featured_media":9982,"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":[2337,884,2343,2338,2344,2339,2335,2340,2341,366,2342,1473,2336,2345,1848],"class_list":["post-8924","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-antiarrhythmic-drugs","tag-arrhythmia","tag-arrhythmia-treatment","tag-cardiac-action-potential","tag-cardiac-electrophysiology","tag-cardiac-pharmacology","tag-class-i-antiarrhythmics","tag-drug-mechanism","tag-fast-sodium-channels","tag-myocardial-infarction","tag-phase-0-depolarization","tag-qrs-complex","tag-sodium-channel-blockers","tag-state-dependent-blockade","tag-ventricular-tachycardia"],"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:\u00a0Class I antiarrhythmics - 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-class-i-antiarrhythmics\" \/>\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:\u00a0Class I antiarrhythmics - Osmosis Blog\" \/>\n<meta property=\"og:description\" content=\"A patient with arrhythmia after myocardial infarction receives a medication that alters phase 0 depolarization. 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