Cardiovascular System MCAT Practice Question
A 58-year-old man with a 10-year history of hypertension presents to the hospital with progressive dyspnea on exertion and orthopnea over the past 3 months. Physical examination reveals elevated jugular venous pressure, a displaced point of maximal impulse, and a third heart sound. Echocardiography shows a left ventricular ejection fraction of 28% with increased left ventricular end-diastolic volume. Cardiac catheterization demonstrates reduced contractility. Genetic testing reveals a heterozygous mutation in the TNNC1 gene resulting in reduced expression of cardiac troponin C. Which of the following mechanisms best explains this patient's impaired myocardial contractility?
Answer choices
- AImpaired calcium buffering by troponin C decreases the total amount of intracellular calcium available for contraction
- BReduced troponin C expression prevents the calcium-induced conformational change required to unblock myosin-binding sites on actinCorrect answer
- CDeficient troponin C increases phosphodiesterase activity, leading to accelerated cAMP degradation and reduced cardiac output
- DDecreased troponin C disrupts Z-disc anchoring proteins, causing progressive sarcomere disorganization and structural heart failure
- EReduced troponin C impairs ATP hydrolysis at the myosin head, decreasing the power stroke force generation
- FTroponin C deficiency prevents sarcoplasmic reticulum calcium release, reducing excitation-contraction coupling efficiency
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