بنك أسئلة USMLE الخطوة الأولى

اقرأ أسئلة عيّنة بنك أسئلة USMLE الخطوة الأولى

1. 1.A 62-year-old man is brought to the emergency department with crushing substernal chest pain radiating to his left arm that began 90 minutes ago. An ECG shows ST-segment elevation in leads II, III, and aVF. Which coronary artery is most likely occluded?

   • أ.Left anterior descending artery
   • ب.Left circumflex artery
   • ج.Right coronary arteryإجابة صحيحة
   • د.Left main coronary artery

   لماذا: ST elevation in the inferior leads (II, III, aVF) indicates an inferior wall myocardial infarction, which in most patients is supplied by the right coronary artery. The RCA also supplies the SA and AV nodes in most individuals, explaining the association with bradyarrhythmias in inferior MI.

2. 2.A 55-year-old woman with a long history of poorly controlled hypertension is found to have concentric left ventricular hypertrophy on echocardiography. At the cellular level, which change best explains the increased ventricular wall thickness?

   • أ.Hyperplasia of cardiac myocytes
   • ب.Addition of sarcomeres in parallel within existing myocytesإجابة صحيحة
   • ج.Addition of sarcomeres in series within existing myocytes
   • د.Interstitial deposition of amyloid protein

   لماذا: Pressure overload from chronic hypertension causes concentric hypertrophy through the addition of sarcomeres in parallel, increasing myocyte width and wall thickness. Cardiac myocytes are terminally differentiated and cannot undergo hyperplasia. Adding sarcomeres in series (volume overload) causes eccentric hypertrophy with chamber dilation.

3. 3.A 24-year-old man collapses during a basketball game and dies. Autopsy reveals asymmetric septal hypertrophy with myofiber disarray. This condition is most commonly caused by a mutation affecting which structure?

   • أ.Sarcomeric proteins such as beta-myosin heavy chainإجابة صحيحة
   • ب.Cardiac sodium channels
   • ج.Dystrophin
   • د.Lamin A/C

   لماذا: Hypertrophic cardiomyopathy is most often caused by autosomal dominant mutations in sarcomeric proteins, especially beta-myosin heavy chain and myosin-binding protein C. The resulting asymmetric septal hypertrophy and myofiber disarray predispose young athletes to sudden cardiac death from ventricular arrhythmias.

4. 4.A 70-year-old man with heart failure is started on a medication that blocks the effects of angiotensin II. He develops a persistent dry cough. Which mechanism best explains this side effect?

   • أ.Decreased aldosterone secretion
   • ب.Reflex bronchoconstriction from hypotension
   • ج.Increased bradykinin levelsإجابة صحيحة
   • د.Direct irritation of bronchial mucosa

   لماذا: ACE inhibitors block angiotensin-converting enzyme, which normally degrades bradykinin. The resulting accumulation of bradykinin causes the characteristic dry cough and can also produce angioedema. This distinguishes ACE inhibitors from angiotensin receptor blockers, which do not raise bradykinin and rarely cause cough.

5. 5.A researcher measures the pressure-volume relationship of a patient's left ventricle. During which phase of the cardiac cycle does the aortic valve open?

   • أ.Isovolumetric contraction
   • ب.At the onset of ventricular ejection when LV pressure exceeds aortic pressureإجابة صحيحة
   • ج.During rapid ventricular filling
   • د.Isovolumetric relaxation

   لماذا: The aortic valve opens when left ventricular pressure rises above aortic pressure at the end of isovolumetric contraction, marking the beginning of the ejection phase. During isovolumetric contraction all valves are closed and volume is constant as pressure builds.

6. 6.A 3-day-old infant develops cyanosis. Echocardiography shows the aorta arising from the right ventricle and the pulmonary artery arising from the left ventricle. This defect results from failure of which embryologic process?

   • أ.Fusion of the endocardial cushions
   • ب.Closure of the ostium secundum
   • ج.Spiraling of the aorticopulmonary septumإجابة صحيحة
   • د.Formation of the ductus arteriosus

   لماذا: Transposition of the great arteries results from failure of the aorticopulmonary (spiral) septum to spiral normally, producing parallel rather than crossing outflow tracts. Survival depends on a shunt such as a patent ductus arteriosus, patent foramen ovale, or VSD to allow mixing of oxygenated and deoxygenated blood.

7. 7.A 68-year-old woman presents with exertional chest pain and syncope. Cardiac examination reveals a harsh crescendo-decrescendo systolic murmur best heard at the right upper sternal border that radiates to the carotids. Which valvular lesion is most likely?

   • أ.Mitral regurgitation
   • ب.Aortic stenosisإجابة صحيحة
   • ج.Aortic regurgitation
   • د.Mitral stenosis

   لماذا: Aortic stenosis produces a crescendo-decrescendo systolic ejection murmur at the right upper sternal border radiating to the carotids. The classic triad of angina, syncope, and heart failure reflects the fixed obstruction to left ventricular outflow. In the elderly it is most often due to calcific degeneration.

8. 8.A 45-year-old man with a history of intravenous drug use develops fever and a new holosystolic murmur at the left lower sternal border that increases with inspiration. Blood cultures grow Staphylococcus aureus. Which valve is most likely affected?

   • أ.Aortic valve
   • ب.Mitral valve
   • ج.Tricuspid valveإجابة صحيحة
   • د.Pulmonic valve

   لماذا: In intravenous drug users, infective endocarditis most commonly affects the tricuspid valve because venous blood carrying organisms reaches the right heart first. Staphylococcus aureus is the most common pathogen. A right-sided murmur classically increases with inspiration (Carvallo sign) due to augmented venous return.

9. 9.A physiologist studies the electrical activity of the SA node. The spontaneous depolarization (phase 4) that gives the SA node its automaticity is primarily due to which ionic current?

   • أ.Rapid inward sodium current through fast sodium channels
   • ب.Outward potassium current
   • ج.Inward calcium current through L-type channels
   • د.Funny current (If) carried by inward sodium flowإجابة صحيحة

   لماذا: The gradual phase 4 depolarization of SA nodal cells is driven largely by the funny current (If), a slow inward flow of sodium through HCN channels activated by hyperpolarization. This spontaneous drift toward threshold underlies the pacemaker automaticity. The upstroke (phase 0) in nodal cells is then carried by L-type calcium current, not fast sodium channels.

10. 10.A 58-year-old man is started on a calcium channel blocker for hypertension. The drug preferentially acts on vascular smooth muscle with little effect on the heart. Which agent was most likely prescribed?

    • أ.Verapamil
    • ب.Diltiazem
    • ج.Amlodipineإجابة صحيحة
    • د.Digoxin

    لماذا: Dihydropyridine calcium channel blockers such as amlodipine act preferentially on vascular smooth muscle, producing vasodilation with minimal direct cardiac effect. Non-dihydropyridines (verapamil, diltiazem) have greater negative inotropic and chronotropic effects on the heart. Digoxin is not a calcium channel blocker.