Contractility determinants US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Contractility determinants. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Contractility determinants US Medical PG Question 1: Which receptor type mediates the slow phase of synaptic transmission in autonomic ganglia?
- A. Muscarinic (M3)
- B. Muscarinic (M2)
- C. Muscarinic (M1) (Correct Answer)
- D. Nicotinic (N2)
Contractility determinants Explanation: ***Muscarinic (M1)***
- **M1 receptors** are **Gq-protein coupled receptors** that activate phospholipase C, leading to increased intracellular calcium and diacylglycerol, which mediates the slow excitatory postsynaptic potential in autonomic ganglia.
- This activation results in a **slow depolarization** that prolongs the excitability of ganglionic neurons after the initial fast synaptic transmission.
*Muscarinic (M3)*
- **M3 receptors** are primarily found on **smooth muscle**, glands, and endothelium, mediating contraction, secretion, and vasodilation, respectively.
- While also **Gq-protein coupled**, their role in autonomic ganglia is not the main mediator of the slow phase of synaptic transmission.
*Muscarinic (M2)*
- **M2 receptors** are **Gi-protein coupled receptors** mainly found in the heart, mediating decreased heart rate and contractility.
- In autonomic ganglia, M2 receptors could have a modulatory role, but they are not responsible for the slow excitatory phase of synaptic transmission.
*Nicotinic (N2)*
- **Nicotinic N2 receptors** (also known as **NN or neuronal nicotinic receptors**) mediate the **fast excitatory postsynaptic potential** (EPSP) in autonomic ganglia by opening ion channels.
- This leads to rapid depolarization and action potential generation, which is distinct from the **slower, prolonged phase** of transmission.
Contractility determinants US Medical PG Question 2: An investigator is developing a drug for muscle spasms. The drug inactivates muscular contraction by blocking the site where calcium ions bind to regulate actin-myosin interaction. Which of the following is the most likely site of action of this drug?
- A. Troponin C (Correct Answer)
- B. Myosin-binding site
- C. Acetylcholine receptor
- D. Ryanodine receptor
- E. Myosin head
Contractility determinants Explanation: ***Troponin C***
- **Calcium ions** bind to **Troponin C**, initiating a conformational change in the troponin-tropomyosin complex, which exposes the **myosin-binding sites on actin**.
- Blocking this site directly prevents the **calcium-mediated regulation** of muscle contraction, thus inactivating it.
*Myosin-binding site*
- The **myosin-binding site** is located on the **actin filament** and is where the **myosin head** attaches to form cross-bridges.
- While essential for contraction, this site doesn't directly bind calcium ions to initiate the process.
*Acetylcholine receptor*
- The **acetylcholine receptor** is located on the **neuromuscular junction** and mediates the transmission of a nerve impulse to the muscle fiber.
- Blocking this receptor would prevent muscle depolarization, but it's not the direct site where calcium ions regulate actin-myosin interaction.
*Ryanodine receptor*
- The **ryanodine receptor** is located on the **sarcoplasmic reticulum** and controls the release of calcium ions into the sarcoplasm.
- While it's involved in calcium signaling, it doesn't represent the site where calcium binds to *regulate* the actin-myosin interaction itself.
*Myosin head*
- The **myosin head** contains the **ATPase activity** and binds to actin to form cross-bridges, enabling muscle contraction.
- It does not directly bind **calcium ions** to regulate the initiation of contraction; instead, its binding to actin is regulated by the troponin-tropomyosin complex.
Contractility determinants US Medical PG Question 3: A 59-year-old male presents to the emergency room complaining of substernal chest pain. He reports a three-hour history of dull substernal chest pain that radiates into his left arm and jaw. He has experienced similar chest pain before that was brought on with exertion, but this pain is more severe and occurred with rest. His past medical history includes gout, hypertension, diabetes mellitus, and hyperlipidemia. An EKG demonstrates ST segment depression. Serum troponin is elevated. In addition to aspirin, oxygen, and morphine, he is started on a sublingual medication. What is the main physiologic effect of this medication?
- A. Decrease preload (Correct Answer)
- B. Decrease heart rate
- C. Increase contractility
- D. Increase preload
- E. Decrease afterload
Contractility determinants Explanation: ***Decrease preload***
- The sublingual medication is likely **nitroglycerin**, which primarily acts as a **venodilator**.
- **Venodilation** leads to pooling of blood in the peripheral veins, thereby reducing the amount of blood returning to the heart, which in turn **decreases preload**.
*Decrease heart rate*
- While medications like **beta-blockers** decrease heart rate, nitroglycerin's primary effect is not heart rate reduction.
- A decreased heart rate reduces myocardial oxygen demand, but this is not the main physiologic effect of sublingual nitroglycerin in an acute setting.
*Increase contractility*
- Increasing contractility would **raise myocardial oxygen demand**, which is counterproductive in a patient experiencing myocardial ischemia.
- Medications that increase contractility, such as **inotropes**, are generally avoided in acute coronary syndromes unless there is severe heart failure.
*Increase preload*
- Increasing preload would **increase myocardial oxygen demand** and potentially exacerbate the patient's ischemic symptoms.
- In acute coronary syndrome, the goal is to reduce cardiac workload to re-establish the balance between oxygen supply and demand.
*Decrease afterload*
- While nitroglycerin can cause some **arterial dilation** and thus decrease afterload, its predominant effect at standard doses for acute chest pain is **venodilation**, leading to a greater reduction in preload.
- Decreasing afterload also helps reduce myocardial oxygen demand but is a secondary effect compared to preload reduction.
Contractility determinants US Medical PG Question 4: A 60-year-old male engineer who complains of shortness of breath when walking a few blocks undergoes a cardiac stress test because of concern for coronary artery disease. During the test he asks his cardiologist about what variables are usually used to quantify the functioning of the heart. He learns that one of these variables is stroke volume. Which of the following scenarios would be most likely to lead to a decrease in stroke volume?
- A. Anxiety
- B. Heart failure (Correct Answer)
- C. Exercise
- D. Pregnancy
- E. Digitalis
Contractility determinants Explanation: ***Heart failure***
- In **heart failure**, the heart's pumping ability is impaired, leading to a reduced **ejection fraction** and thus a decreased **stroke volume**.
- The weakened myocardium cannot effectively contract to expel the normal volume of blood, resulting in lower blood output per beat.
*Anxiety*
- **Anxiety** typically causes an increase in **sympathetic nervous system** activity, leading to increased heart rate and myocardial contractility.
- This often results in a temporary **increase in stroke volume** due to enhanced cardiac performance, not a decrease.
*Exercise*
- During **exercise**, there is a significant **increase in venous return** and sympathetic stimulation, leading to increased **end-diastolic volume** and contractility.
- This physiological response causes a substantial **increase in stroke volume** to meet the body's higher oxygen demands.
*Pregnancy*
- **Pregnancy** leads to significant **physiological adaptations** to accommodate the growing fetus, including a substantial increase in **blood volume**.
- This increased blood volume and cardiac output result in an **increase in stroke volume** to maintain adequate perfusion for both mother and fetus.
*Digitalis*
- **Digitalis** is a cardiac glycoside that **increases intracellular calcium** in myocardial cells, enhancing the **force of contraction**.
- This positive inotropic effect leads to an **increased stroke volume** by improving the heart's pumping efficiency.
Contractility determinants US Medical PG Question 5: An investigator is studying the physiological response during congestive heart failure exacerbations in patients with systolic heart failure. A hormone released by ventricular cardiomyocytes in response to increased wall stress is isolated from a patient's blood sample. The intracellular mechanism by which this hormone acts is most similar to the effect of which of the following substances?
- A. Aldosterone
- B. Platelet-derived growth factor
- C. Nitric oxide (Correct Answer)
- D. Angiotensin II
- E. Human chorionic gonadotropin
Contractility determinants Explanation: ***Nitric oxide***
- The hormone described is likely **Brain Natriuretic Peptide (BNP)**, released from ventricular cardiomyocytes in response to **increased wall stress** during heart failure. BNP acts by increasing intracellular **cGMP** to cause vasodilation and natriuresis.
- **Nitric oxide (NO)** also works by stimulating **guanylyl cyclase** to increase intracellular cGMP, leading to smooth muscle relaxation and vasodilation, thus mimicking the intracellular mechanism of BNP.
*Aldosterone*
- **Aldosterone** is a steroid hormone that acts on intracellular mineralocorticoid receptors, altering gene expression to promote **sodium reabsorption** and potassium excretion.
- This mechanism is distinct from the **second messenger system** involving cGMP.
*Platelet-derived growth factor*
- **Platelet-derived growth factor (PDGF)** binds to cell surface receptor tyrosine kinases, initiating a signaling cascade that primarily involves **phosphorylation** and leads to cell growth and proliferation.
- This mechanism is different from the direct activation of **guanylyl cyclase** by BNP.
*Angiotensin II*
- **Angiotensin II** primarily acts on G protein-coupled receptors (GPCRs), leading to vasoconstriction, aldosterone release, and other effects, often through **IP3/DAG** or **cAMP** pathways.
- Its intracellular signaling mechanism is not primarily centered on **cGMP**.
*Human chorionic gonadotropin*
- **Human chorionic gonadotropin (hCG)** is a glycoprotein hormone that binds to **G protein-coupled receptors (GPCRs)**, primarily stimulating the production of **cAMP** as a second messenger.
- While it uses a second messenger system, the specific pathway (cAMP) differs from the **cGMP** pathway activated by BNP.
Contractility determinants US Medical PG Question 6: A previously healthy 52-year-old woman comes to the physician because of a 3-month history of chest pain on exertion. She takes no medications. Cardiopulmonary examination shows no abnormalities. Cardiac stress ECG shows inducible ST-segment depressions in the precordial leads that coincide with the patient's report of chest pain and resolve upon cessation of exercise. Pharmacotherapy with verapamil is initiated. This drug is most likely to have which of the following sets of effects?
$$$ End-diastolic volume (EDV) %%% Blood pressure (BP) %%% Contractility %%% Heart rate (HR) $$$
- A. No change no change no change no change
- B. ↓ ↓ no change ↑
- C. ↓ ↓ ↓ ↑
- D. ↓ ↓ ↓ no change
- E. ↑ ↓ ↓ ↓ (Correct Answer)
Contractility determinants Explanation: ***↑ ↓ ↓ ↓***
- **Verapamil**, a **non-dihydropyridine calcium channel blocker**, reduces **cardiac contractility**, leading to decreased **heart rate** and **blood pressure**, while increasing **end-diastolic volume**.
- Its therapeutic effect in **exertional angina** is primarily due to reduced myocardial oxygen demand, achieved by decreasing **heart rate**, **contractility** (both leading to reduced work of heart), and **afterload** (due to vasodilation which decreases blood pressure).
*No change no change no change no change*
- This option is incorrect because verapamil has significant **pharmacological effects** on the cardiovascular system.
- Verapamil is prescribed to treat the patient's symptoms, implying a need for **hemodynamic changes**, not stasis.
*↓ ↓ no change ↑*
- Verapamil typically **decreases heart rate** due to its action on the sinoatrial (SA) node, making an increase unlikely.
- While it decreases **blood pressure** and **contractility**, the absence of an effect on heart rate and an increase in heart rate are inconsistent with verapamil's known pharmacology.
*↓ ↓ ↓ ↑*
- This option incorrectly suggests an **increase in heart rate**, whereas verapamil is known to cause a dose-dependent **decrease in heart rate**.
- The other parameters (decreased EDV, BP, contractility) are also not fully aligned with verapamil's effects; EDV tends to increase due to better filling time and reduced contractility.
*↓ ↓ ↓ no change*
- This option suggests a **decrease in EDV**, which is generally incorrect; verapamil tends to allow for **increased ventricular filling** due to a reduced heart rate and prolonged diastole.
- The absence of a change in heart rate is also incorrect, as verapamil is a known **negative chronotropic agent**.
Contractility determinants US Medical PG Question 7: Which neurotransmitter is primarily responsible for parasympathetic effects on heart rate?
- A. Norepinephrine
- B. Dopamine
- C. Acetylcholine (Correct Answer)
- D. Epinephrine
Contractility determinants Explanation: ***Acetylcholine***
- **Acetylcholine** is the primary neurotransmitter released by postganglionic parasympathetic neurons.
- It acts on **muscarinic receptors** (M2 receptors) in the heart to decrease heart rate.
*Norepinephrine*
- **Norepinephrine** is primarily associated with the **sympathetic nervous system**, increasing heart rate and contractility.
- It acts on **beta-1 adrenergic receptors** in the heart.
*Dopamine*
- **Dopamine** is a precursor to norepinephrine and epinephrine, and primarily functions as a neurotransmitter in the **central nervous system** and in regulating renal blood flow.
- While it can have cardiac effects, it is not the primary neurotransmitter for parasympathetic actions on heart rate.
*Epinephrine*
- **Epinephrine** (adrenaline) is a hormone released by the adrenal medulla and a neurotransmitter in the sympathetic nervous system, causing an **increase in heart rate** and contractility.
- It works through **beta-1 adrenergic receptors**, antagonistic to parasympathetic effects.
Contractility determinants US Medical PG Question 8: A 62-year-old man is brought to the emergency department because of a 4-hour history of abdominal pain, nausea, vomiting, and confusion. His wife reports that he had blurry vision on the way to the hospital. Two weeks ago, he lost his job and since then has been extremely worried about their financial situation and future. He has congestive heart failure and atrial fibrillation well controlled with combination medical therapy. His temperature is 36.5°C (97.7°F), pulse is 57/min and irregular, respirations are 14/min, and blood pressure is 118/63 mm Hg. The patient is oriented only to person. Serum studies show:
Na+ 138 mEq/L
Cl− 100 mEq/L
K+ 5.3 mEq/L
HCO3− 25 mEq/L
Blood urea nitrogen 14 mg/dL
Creatinine 0.9 mg/dL
An ECG shows premature ventricular beats. The drug most likely responsible for this patient's symptoms has which of the following mechanisms of action?
- A. Blockade of aldosterone receptors
- B. Blockade of beta-adrenergic receptors
- C. Inhibition of Na+/K+-ATPase (Correct Answer)
- D. Inhibition of Na+-K+-2Cl--cotransporters
- E. Inhibition of funny channels
Contractility determinants Explanation: ***Inhibition of Na+/K+-ATPase***
- The patient's symptoms (confusion, blurry vision, nausea, vomiting, abdominal pain, arrhythmia, hyperkalemia) are classic for **digoxin toxicity**, which occurs due to the inhibition of the **Na+/K+-ATPase pump**.
- Inhibition of this pump leads to increased intracellular calcium, enhancing cardiac contractility but also causing hyperexcitability and arrhythmias like **premature ventricular beats**.
*Blockade of aldosterone receptors*
- This mechanism is characteristic of **aldosterone antagonists** (e.g., spironolactone, eplerenone) which are often used in heart failure.
- While they can cause hyperkalemia, they typically do not cause the constellation of neurological (confusion, blurry vision) and gastrointestinal symptoms seen in this patient.
*Blockade of beta-adrenergic receptors*
- This is the mechanism of **beta-blockers** (e.g., carvedilol, metoprolol), also commonly used in heart failure and atrial fibrillation.
- Symptoms of beta-blocker overdose usually include bradycardia, hypotension, and bronchospasm, but not the prominent GI or blurry vision symptoms seen here.
*Inhibition of Na+-K+-2Cl--cotransporters*
- This mechanism belongs to **loop diuretics** (e.g., furosemide), often used in congestive heart failure.
- Loop diuretics primarily cause electrolyte imbalances such as hypokalemia and hypomagnesemia, and volume depletion, which does not match the patient's presentation of hyperkalemia and specific digoxin toxicity symptoms.
*Inhibition of funny channels*
- This is the mechanism of action of **ivabradine**, a selective inhibitor of the I_f current in the sinoatrial node, used to reduce heart rate in heart failure.
- While it can cause bradycardia, it is not associated with the severe GI distress, neurological symptoms, or hyperkalemia observed in this patient.
Contractility determinants US Medical PG Question 9: A 68-year-old male with a history of congestive heart failure presents to his cardiologist complaining of mild dyspnea on exertion and swollen ankles. His past medical history is also significant for hypertension and alcohol abuse. He has a 50 pack-year smoking history. He currently takes lisinopril, aspirin, and metoprolol. His temperature is 99°F (37.2°C), blood pressure is 135/85 mmHg, pulse is 85/min, and respirations are 18/min. An echocardiogram reveals an ejection fraction of 35%. His cardiologist adds an additional medication to the patient’s regimen. Two weeks later, the patient notices yellow halos in his vision. Which of the following medications did this patient most likely start taking?
- A. Dobutamine
- B. Nitroprusside
- C. Hydralazine
- D. Digoxin (Correct Answer)
- E. Furosemide
Contractility determinants Explanation: ***Digoxin***
- The patient's symptoms (dyspnea on exertion, swollen ankles, reduced ejection fraction) indicate **heart failure**. **Digoxin** is a cardiac glycoside used to improve cardiac output in heart failure.
- **Yellow halos in vision** are a classic symptom of **digoxin toxicity**, which can occur if drug levels become too high.
*Dobutamine*
- **Dobutamine** is a **beta-1 adrenergic agonist** used as an inotrope in acute decompensated heart failure to improve cardiac contractility.
- It is typically administered intravenously for short-term management and is not associated with yellow halos in vision.
*Nitroprusside*
- **Nitroprusside** is a potent **vasodilator** used for hypertensive emergencies and in some cases of acute decompensated heart failure to reduce preload and afterload.
- It is also administered intravenously and is not associated with the visual disturbances described.
*Hydralazine*
- **Hydralazine** is a direct **arterial vasodilator** used to reduce afterload in heart failure, often in combination with nitrates. It can cause reflex tachycardia.
- While used in chronic heart failure, it does not typically cause visual changes such as yellow halos.
*Furosemide*
- **Furosemide** is a **loop diuretic** used to reduce fluid overload in heart failure, alleviating symptoms like edema and dyspnea.
- While it can cause ototoxicity at high doses or in renal impairment, it does not cause yellow halos in vision.
Contractility determinants US Medical PG Question 10: You are interested in studying the etiology of heart failure reduced ejection fraction (HFrEF) and attempt to construct an appropriate design study. Specifically, you wish to look for potential causality between dietary glucose consumption and HFrEF. Which of the following study designs would allow you to assess for and determine this causality?
- A. Cross-sectional study
- B. Case series
- C. Cohort study (Correct Answer)
- D. Case-control study
- E. Randomized controlled trial
Contractility determinants Explanation: ***Cohort study***
- A **cohort study** observes a group of individuals over time to identify risk factors and outcomes, allowing for the assessment of **temporal relationships** between exposure (dietary glucose) and outcome (HFrEF).
- This design is suitable for establishing a potential **causal link** as it tracks participants from exposure to outcome, enabling the calculation of incidence rates and relative risks.
*Cross-sectional study*
- A **cross-sectional study** measures exposure and outcome simultaneously at a single point in time, making it impossible to determine the **temporal sequence** of events.
- This design can only identify **associations** or correlations, not causation, as it cannot establish whether high glucose consumption preceded HFrEF.
*Case series*
- A **case series** describes characteristics of a group of patients with a particular disease or exposure, often to highlight unusual clinical features, but it lacks a **comparison group**.
- It cannot assess causality because it does not provide information on the frequency of exposure in healthy individuals or the incidence of the disease in unexposed individuals.
*Case-control study*
- A **case-control study** compares individuals with the outcome (cases) to those without the outcome (controls) to determine past exposures, which makes it prone to **recall bias**.
- While it can suggest associations, it cannot definitively establish a temporal relationship or causation as the outcome is already known when exposure is assessed.
*Randomized controlled trial*
- A **randomized controlled trial (RCT)** is the gold standard for establishing causation by randomly assigning participants to an intervention or control group, but it may not be ethical or feasible for studying long-term dietary exposures and chronic diseases like HFrEF due to the long follow-up period and complexity of diet.
- While ideal for causality, directly controlling and randomizing dietary glucose intake over decades to observe HFrEF development might be practically challenging or unethical.
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