USMLE Step 1 Medical Prep Exam Questions & Answers 2026 (21–30)

Question 1

Which of the following accurately describes the function of cardiac troponin I?

TheoryPractice · Accepted Answer

Inhibits actin-myosin interaction in the absence of calcium. Cardiac troponin I is a regulatory protein found in cardiac muscle that inhibits actin-myosin interactions in the absence of calcium. When calcium binds to troponin C, troponin I shifts position, allowing contraction to occur. Elevated levels in blood indicate cardiac muscle damage, making it a specific marker for myocardial injury.

Question 2

A 55-year-old man with hypertension and hyperlipidemia presents with acute chest pain. His ECG shows ST-segment depression in leads V1-V4. Which type of acute coronary syndrome is most likely?

TheoryPractice · Accepted Answer

Non-ST elevation myocardial infarction (NSTEMI). ST-segment depression in the absence of ST elevation is characteristic of non-ST elevation myocardial infarction (NSTEMI) or unstable angina, collectively referred to as non-ST elevation acute coronary syndrome (NSTE-ACS). This typically results from partial coronary occlusion or transient complete occlusion, in contrast to STEMI, which shows ST elevation due to complete occlusion.

Question 3

Which of the following medications used in heart failure acts by inhibiting sodium-potassium ATPase?

TheoryPractice · Accepted Answer

Digoxin. Digoxin inhibits the sodium-potassium ATPase pump in cardiac myocytes, leading to increased intracellular sodium. This promotes increased calcium entry via the sodium-calcium exchanger, resulting in positive inotropy (increased contractility). Digoxin is used in heart failure to improve cardiac output and in atrial fibrillation to control ventricular rate.

Question 4

A 42-year-old woman presents with recurrent episodes of dizziness and syncope. ECG during an episode shows a prolonged QT interval of 520 ms. Which electrolyte abnormality is most likely to exacerbate this condition?

TheoryPractice · Accepted Answer

Hypokalemia. Hypokalemia prolongs the QT interval by delaying ventricular repolarization and is particularly dangerous in patients with congenital long QT syndrome. Low potassium levels can trigger torsades de pointes (a form of polymorphic ventricular tachycardia), which can lead to syncope or sudden cardiac death.

Question 5

In the cardiac cycle, which event corresponds to the onset of the first heart sound (S1)?

TheoryPractice · Accepted Answer

Closure of the atrioventricular valves. The first heart sound (S1) corresponds to the closure of the atrioventricular valves (mitral and tricuspid) at the beginning of ventricular systole. This occurs when ventricular pressure exceeds atrial pressure, causing the AV valves to snap shut, creating the 'lub' of 'lub-dub.'

Question 6

A 30-year-old woman has a blood pressure of 100/60 mmHg. What is her calculated mean arterial pressure (MAP)?

TheoryPractice · Accepted Answer

73 mmHg. Mean arterial pressure (MAP) is calculated as: MAP = Diastolic pressure + (1/3 × Pulse pressure). Where pulse pressure = Systolic pressure - Diastolic pressure. MAP = 60 + (1/3 × (100-60)) = 60 + (1/3 × 40) = 60 + 13.3 ≈ 73 mmHg.

Question 7

Which of the following vessel layers is primarily responsible for maintaining vascular tone?

TheoryPractice · Accepted Answer

Tunica media. The tunica media (middle layer) of blood vessels contains smooth muscle cells and elastic fibers that control vessel diameter through contraction and relaxation. This layer is primarily responsible for maintaining vascular tone, which is crucial for regulating blood pressure and regional blood flow.

Question 8

A 68-year-old man with a history of myocardial infarction is started on lisinopril. Which of the following describes the primary mechanism of this medication?

TheoryPractice · Accepted Answer

Inhibition of angiotensin-converting enzyme. Lisinopril is an angiotensin-converting enzyme (ACE) inhibitor that prevents the conversion of angiotensin I to angiotensin II. This leads to decreased aldosterone secretion, decreased vasoconstriction, and overall reduction in blood pressure. ACE inhibitors are beneficial in post-MI patients by reducing afterload and preventing adverse cardiac remodeling.

Question 9

A 57-year-old man with a 40-pack-year smoking history presents with leg pain when walking that resolves with rest. The pain is described as cramping in the calves. Which condition is most likely present?

TheoryPractice · Accepted Answer

Peripheral arterial disease. This patient presents with classic symptoms of intermittent claudication, which is exercise-induced ischemic muscle pain that resolves with rest. It is typically caused by peripheral arterial disease (PAD), most commonly due to atherosclerosis. Smoking is a major risk factor for PAD, and the calf muscles are commonly affected due to their high oxygen demand during walking.

Question 10

Which of the following cells forms the majority of the cardiac conduction system?

TheoryPractice · Accepted Answer

Purkinje fibers. Purkinje fibers form the majority of the specialized cardiac conduction system. They originate from the bundle of His and rapidly conduct electrical impulses throughout the ventricles, facilitating synchronized ventricular contraction. They have fewer myofibrils and more glycogen than regular cardiomyocytes, and conduct impulses about 4 times faster.

USMLE Step 1 Medical Prep Exam Questions & Answers 2026 (21–30)

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