NBME Comprehensive Basic Science 3 — Questions and Answers
Question 1: A 55-year-old man develops digitalis toxicity. Which electrolyte abnormality most potentiates this toxicity?
- Hypokalemia (Correct answer)
- Hyperkalemia
- Hyponatremia
- Hypermagnesemia
Correct answer: Hypokalemia
Hypokalemia potentiates digoxin toxicity because K+ and digoxin compete for the same binding site on the Na+/K+-ATPase.
Question 2: The Frank-Starling mechanism states that increased ventricular end-diastolic volume leads to which of the following?
- Increased stroke volume (Correct answer)
- Decreased cardiac output
- Reduced preload
- Increased afterload
Correct answer: Increased stroke volume
Greater sarcomere stretch from increased preload optimizes actin-myosin overlap, increasing the force of contraction and stroke volume.
Question 3: A patient with renal artery stenosis has elevated renin. Which of the following is the direct product of renin's enzymatic action?
- Angiotensin I (Correct answer)
- Angiotensin II
- Aldosterone
- Bradykinin
Correct answer: Angiotensin I
Renin cleaves angiotensinogen to angiotensin I, which is then converted to angiotensin II by ACE in the lungs.
Question 4: A drug inhibits cyclooxygenase irreversibly. Which cell type is most functionally impaired because it cannot synthesize new enzyme?
- Platelets (Correct answer)
- Neutrophils
- Macrophages
- Endothelial cells
Correct answer: Platelets
Platelets are anucleate and cannot synthesize new COX; aspirin's irreversible inhibition affects them for their entire 10-day lifespan.
Question 5: During a Valsalva maneuver, which phase is associated with a transient increase in venous return and cardiac output upon release?
- Phase IV (post-release overshoot) (Correct answer)
- Phase I (onset of straining)
- Phase II (continued straining)
- Phase III (immediate release)
Correct answer: Phase IV (post-release overshoot)
Phase IV shows reflex bradycardia and BP overshoot as thoracic venous blood surges back to the heart after intra-thoracic pressure normalizes.
Question 6: A patient taking a loop diuretic develops metabolic alkalosis. Which mechanism best explains this?
- Contraction alkalosis and increased aldosterone-mediated H+ secretion (Correct answer)
- Decreased renal bicarbonate reabsorption
- Hyperventilation-induced CO2 retention
- Increased lactic acid production
Correct answer: Contraction alkalosis and increased aldosterone-mediated H+ secretion
Volume depletion concentrates bicarbonate (contraction alkalosis) and stimulates aldosterone, increasing collecting duct H+ secretion.
Question 7: A newborn is found to have a patent ductus arteriosus. Which auscultatory finding is most characteristic?
- Continuous machine-like murmur at the left infraclavicular area (Correct answer)
- Mid-systolic click followed by a late systolic murmur
- Fixed split S2 with ejection systolic murmur
- Early diastolic decrescendo murmur at the left sternal border
Correct answer: Continuous machine-like murmur at the left infraclavicular area
The PDA allows continuous left-to-right shunting throughout systole and diastole, producing a machinery murmur heard best below the left clavicle.
NBME Comprehensive Basic Science 3
A 55-year-old man develops digitalis toxicity.
Which electrolyte abnormality most potentiates this toxicity?