A 21-year-old man presents to the physician with numbness and weakness in both legs for about a day. He also mentions that both thighs ache. His past medical history is not significant except for some diarrhea about 3 weeks ago. On physical examination, his temperature is 37.2ºC (99.0ºF), pulse rate is 108/min and respiratory rate is 14/min. His blood pressure is 122/82 mm Hg and 100/78 mm Hg in supine and upright positions, respectively. His neurologic evaluation reveals the presence of bilateral symmetrical weakness of the lower limbs, the absence of deep tendon reflexes, and negative Babinski sign. His sensorium and higher brain functions are normal. Which of the following options best explains the pathophysiological mechanism underlying the patient’s clinical features?
Q122
In a patient with acute myocardial ischemia, which of the following cardiovascular structures is at greatest risk of damage?
Q123
A 66-year-old male presents to the emergency room with shortness of breath with exertion and at rest for the past 5 days. His shortness of breath is mostly at night, and he is also concerned about bilateral leg swelling. He is a heart failure patient who is being managed with oral medication and has been compliant with his drugs. Physical examination reveals an elderly man in respiratory distress with abdominal distention and bilateral pitting ankle edema. Respiratory rate is 32/min, SpO2 is 93% in room air, and coarse crepitations are heard on both lung bases. Pulse rate is 73/min and barely palpable. His blood pressure is 79/54 mm Hg. On auscultation, a blowing holosystolic murmur is heard at the apex radiating to the left axilla. An echocardiography shows an ejection fraction of 18%. The physician decides to include an inotropic agent in his current medication. What would likely result from this intervention?
Q124
A 70-year-old woman presents to her primary care physician with sudden episodes of dizziness that resolve in certain positions. On further questioning she describes a false sense of motion with occasional spinning sensation consistent with vertigo. She denies any recent illnesses or hearing loss aside from presbycusis. Her vital signs are normal. During the physical exam the patient reports an episode of vertigo after transitioning from sitting to supine and horizontal nystagmus is concurrently noted. What is the most likely diagnosis?
Q125
A 71-year-old man with hypertension comes to the physician for a follow-up examination. Cardiovascular exam shows the point of maximal impulse to be in the mid-axillary line. A transthoracic echocardiogram shows concentric left ventricular hypertrophy with a normal right ventricle. Which of the following is the most likely underlying mechanism of this patient's ventricular hypertrophy?
Q126
A 49-year-old woman presents to the office because of tremors for 2 months. She says that her hands have been shaking a lot, especially when she feels stressed. In addition, she has been sweating more than usual and has lost 8 kg (17.6 lb) in the last 2 months. She has a past medical history of vitiligo. Her vital signs are a heart rate of 98/min, a respiratory rate of 14/min, a temperature of 37.6°C (99.7°F), and a blood pressure of 115/75 mm Hg. Physical examination shows a fine, bilateral hand tremor and a diffuse goiter. Which of the following hormonal imbalances is most likely present?
Q127
A previously healthy 57-year-old man comes to the emergency department because of acute retrosternal chest pain that radiates to his back. The pain started suddenly while he was having dinner. A few moments prior to the onset of the pain, he experienced discomfort when trying to eat or drink anything. On the way to the hospital he took a sublingual nitrate tablet that he had at home, which helped relieve the pain. His pulse is 80/min, respirations are 14/min, and blood pressure is 144/88 mm Hg. Examination shows no other abnormalities. An ECG shows a normal sinus rhythm with no ST-segment abnormalities. An esophagogram is done and shows areas of diffuse, uncoordinated spasms in several segments along the length of the esophagus. This patient's condition is most likely to show which of the following findings?
Q128
Nine healthy subjects participate in a study of gastric secretions. Subjects are asked to eat a meal at hour 0, at which time the pH of stomach contents and rate of stomach acid secretions are measured over the next 4 hours. Results of the study are shown. Which of the following mediators is most active at point A in the graph?
Q129
An investigator is studying cardiomyocytes in both normal and genetically modified mice. Both groups are observed after aerobic exercise and their heart rates are recorded and compared. After a 10-minute session on a treadmill, the average pulse measured in the normal mice is 680/min, whereas in the genetically modified mice it is only 160/min. Which of the following is most likely to account for the normal mice's ability to achieve higher heart rates during exercise stress?
Q130
A 33-year-old female presents to her primary care physician complaining of heat intolerance and difficulty sleeping over a one month period. She also reports that she has lost 10 pounds despite no changes in her diet or exercise pattern. More recently, she has developed occasional unprovoked chest pain and palpitations. Physical examination reveals a nontender, mildly enlarged thyroid gland. Her patellar reflexes are 3+ bilaterally. Her temperature is 99°F (37.2°C), blood pressure is 135/85 mmHg, pulse is 105/min, and respirations are 18/min. Laboratory analysis is notable for decreased TSH. Which of the following pathophysiologic mechanisms contributed to the cardiovascular symptoms seen in this patient?
Cardiovascular US Medical PG Practice Questions and MCQs
Question 121: A 21-year-old man presents to the physician with numbness and weakness in both legs for about a day. He also mentions that both thighs ache. His past medical history is not significant except for some diarrhea about 3 weeks ago. On physical examination, his temperature is 37.2ºC (99.0ºF), pulse rate is 108/min and respiratory rate is 14/min. His blood pressure is 122/82 mm Hg and 100/78 mm Hg in supine and upright positions, respectively. His neurologic evaluation reveals the presence of bilateral symmetrical weakness of the lower limbs, the absence of deep tendon reflexes, and negative Babinski sign. His sensorium and higher brain functions are normal. Which of the following options best explains the pathophysiological mechanism underlying the patient’s clinical features?
A. Autoantibody-mediated destruction of Ca2+ channels in the nerve endings at the neuromuscular junction
B. Decreased neuronal excitability (Correct Answer)
C. Genetic overexpression of K+ channels in skeletal muscle
D. Decreased threshold for the generation and propagation of the action potential
E. Toxin-mediated blockade of voltage-gated fast Na+ channels
Explanation: ***Decreased neuronal excitability***
- This patient's presentation with **ascending paralysis**, **areflexia**, and a history of antecedent infection (diarrhea) is highly suggestive of **Guillain-Barré Syndrome (GBS)**.
- GBS involves **demyelination** of peripheral nerves, leading to decreased conduction velocity and, consequently, decreased neuronal excitability and signal transmission.
*Autoantibody-mediated destruction of Ca2+ channels in the nerve endings at the neuromuscular junction*
- This mechanism describes **Lambert-Eaton Myasthenic Syndrome (LEMS)**, which typically presents with proximal muscle weakness that improves with activity and is often associated with small cell lung cancer.
- Unlike GBS, LEMS primarily affects the **presynaptic neuromuscular junction**, and the weakness usually *improves* with repetitive stimulation, contrasting with the progressive weakness seen here.
*Genetic overexpression of K+ channels in skeletal muscle*
- **Overexpression of K+ channels** in skeletal muscle is not a recognized direct cause of acute ascending paralysis or GBS.
- This mechanism is more relevant to certain channelopathies causing episodic muscle weakness or paralysis, distinct from the autoimmune demyelination seen in GBS.
*Decreased threshold for the generation and propagation of the action potential*
- A decreased threshold implies **hyperexcitability** or easier generation of action potentials, which would typically lead to symptoms like muscle spasms, cramps, or even seizures.
- This is the opposite of what is observed in GBS, where nerve conduction is impaired, leading to **weakness and numbness**.
*Toxin-mediated blockade of voltage-gated fast Na+ channels*
- **Toxin-mediated blockade of Na+ channels** can certainly cause paralysis (e.g., tetrodotoxin, saxitoxin) by preventing action potential generation.
- However, the clinical context of antecedent diarrhea and ascending paralysis points much more strongly to an autoimmune demyelinating condition like GBS rather than an acute poisoning.
Question 122: In a patient with acute myocardial ischemia, which of the following cardiovascular structures is at greatest risk of damage?
A. Pulmonary valve
B. Cardiac conduction system (Correct Answer)
C. Coronary artery
D. Cardiac septum
E. Temporal artery
Explanation: ***Cardiac conduction system***
- The **cardiac conduction system** is highly dependent on a constant oxygen supply, and its disruption by ischemia can lead to serious **arrhythmias** and **heart blocks**.
- Ischemia in critical areas like the **AV node** (supplied by the RCA) or the **bundle branches** can severely impair the heart's electrical activity.
*Pulmonary valve*
- The **pulmonary valve** is primarily a passive structure and is generally not directly damaged by acute myocardial ischemia.
- Its function is more affected by changes in **pulmonary artery pressure** or **ventricular remodeling**, not immediate ischemic injury.
*Coronary artery*
- While **coronary artery disease (CAD)** is the *cause* of myocardial ischemia, the coronary artery itself is not the structure *damaged* in the sense of functional impairment due to lack of blood flow in acute ischemia.
- The damage occurs downstream in the **myocardium** that the artery supplies.
*Cardiac septum*
- The **cardiac septum** can be damaged by myocardial ischemia, particularly the **interventricular septum**, leading to complications like **septal rupture**.
- However, the conduction system is at *greatest* immediate risk of functional damage leading to life-threatening events due to its critical role in rhythm generation.
*Temporal artery*
- The **temporal artery** is a blood vessel located in the head, entirely separate from the heart.
- It is not involved in myocardial ischemia and is not at risk of damage from a cardiac event.
Question 123: A 66-year-old male presents to the emergency room with shortness of breath with exertion and at rest for the past 5 days. His shortness of breath is mostly at night, and he is also concerned about bilateral leg swelling. He is a heart failure patient who is being managed with oral medication and has been compliant with his drugs. Physical examination reveals an elderly man in respiratory distress with abdominal distention and bilateral pitting ankle edema. Respiratory rate is 32/min, SpO2 is 93% in room air, and coarse crepitations are heard on both lung bases. Pulse rate is 73/min and barely palpable. His blood pressure is 79/54 mm Hg. On auscultation, a blowing holosystolic murmur is heard at the apex radiating to the left axilla. An echocardiography shows an ejection fraction of 18%. The physician decides to include an inotropic agent in his current medication. What would likely result from this intervention?
A. A decrease in the interval between the heart sounds S2 and S1
B. A decrease in the systemic vascular resistance
C. A decrease in the left ventricular end-diastolic pressure
D. A decrease in the interval between the heart sounds S1 and S2 (Correct Answer)
E. An increase in the left ventricular end-systolic volume
Explanation: ***A decrease in the interval between the heart sounds S1 and S2***
- The patient has severe **heart failure with reduced ejection fraction** (HFrEF) and likely cardiogenic shock, indicated by **hypotension** and **respiratory distress**. The **holosystolic murmur at the apex radiating to the left axilla** suggests **mitral regurgitation**, exacerbating his condition.
- An **inotropic agent** (e.g., dobutamine) increases myocardial contractility, leading to a **more forceful and faster ventricular ejection**. This causes the **aortic valve to close earlier**, effectively **shortening systole** and thus the interval between S1 (mitral valve closure) and S2 (aortic valve closure).
*A decrease in the interval between the heart sounds S2 and S1*
- The interval between S2 and S1 represents **diastole**. An inotropic agent primarily affects **systolic function**, and while improved cardiac output can influence diastolic filling times, a primary and direct decrease in the S2-S1 interval is not the expected or most significant effect.
- In cases of severe heart failure, changes to diastolic timing are complex and secondary to changes in systolic function and filling pressures, but not a direct result of inotropic therapy in this manner.
*A decrease in the systemic vascular resistance*
- While some inotropic agents like **dobutamine** have **beta-2 agonist activity** that can cause some vasodilation and thus a decrease in SVR, their primary effect is to **increase contractility**.
- Other inotropes, like **norepinephrine**, can actually **increase SVR**. The main therapeutic goal here is to improve cardiac output, and a direct and primary decrease in SVR isn't the guaranteed or most significant outcome from the inotropic effect itself.
*A decrease in the left ventricular end-diastolic pressure*
- An inotropic agent increases cardiac output, which can lead to better systemic perfusion and **reduce venous congestion** over time. However, in the acute setting of severe heart failure with volume overload, **LVEDP** is initially high due to impaired relaxation and increased preload.
- While improved contractility might lead to more effective forward flow, it does not directly or immediately decrease LVEDP; rather, this is often achieved through diuretics or vasodilators. In some cases, increased contractility itself can even transiently increase LVEDP if there's significant myocardial stiffness.
*An increase in the left ventricular end-systolic volume*
- An inotropic agent directly **enhances myocardial contractility**, allowing the left ventricle to **eject more blood** with each beat.
- This increased contractility leads to a **more complete emptying of the left ventricle** during systole, resulting in a **decrease** (not an increase) in the **left ventricular end-systolic volume** (LVESV).
Question 124: A 70-year-old woman presents to her primary care physician with sudden episodes of dizziness that resolve in certain positions. On further questioning she describes a false sense of motion with occasional spinning sensation consistent with vertigo. She denies any recent illnesses or hearing loss aside from presbycusis. Her vital signs are normal. During the physical exam the patient reports an episode of vertigo after transitioning from sitting to supine and horizontal nystagmus is concurrently noted. What is the most likely diagnosis?
A. Vestibular migraine
B. Vestibular neuritis
C. Benign Paroxysmal Positional Vertigo (BPPV) (Correct Answer)
D. Meniere's disease
E. Labyrinthitis
Explanation: ***Benign Paroxysmal Positional Vertigo (BPPV)***
- The presentation of sudden **positional vertigo** and **positional nystagmus** (triggered by changing from sitting to supine) in an elderly patient is classic for BPPV.
- BPPV is caused by displaced **otoconia** within the semicircular canals, which inappropriately stimulate hair cells with head movements.
*Vestibular migraine*
- This condition involves recurrent vertigo attacks often associated with **migrainous headaches** or other migraine features (photophobia, phonophobia), which are not described here.
- While vertigo can be positional, the lack of headache or migraine history makes BPPV a more likely diagnosis.
*Vestibular neuritis*
- Typically presents with a **sudden onset of severe vertigo** that is constant and can last for days, often accompanied by **nausea and vomiting**, but without hearing loss.
- The vertigo in this case is *episodic* and *positional*, which does not fit the typical pattern of vestibular neuritis.
*Meniere's disease*
- Characterized by a classic triad of **episodic vertigo, fluctuating sensorineural hearing loss, and tinnitus**, often with aural fullness.
- While vertigo attacks can be sudden, the absence of **tinnitus** or *significant* hearing loss (beyond age-related presbycusis) makes Meniere's less likely.
*Labyrinthitis*
- Involves inflammation of the inner ear, presenting with **sudden, severe vertigo, nausea, and vomiting**, similar to vestibular neuritis, but also includes **unilateral hearing loss**.
- The patient specifically denies recent illnesses and significant hearing loss, which rules out labyrinthitis.
Question 125: A 71-year-old man with hypertension comes to the physician for a follow-up examination. Cardiovascular exam shows the point of maximal impulse to be in the mid-axillary line. A transthoracic echocardiogram shows concentric left ventricular hypertrophy with a normal right ventricle. Which of the following is the most likely underlying mechanism of this patient's ventricular hypertrophy?
A. Accumulation of protein fibrils
B. Accumulation of glycogen
C. Infiltration of T lymphocytes
D. Deposition of endomyocardial collagen
E. Accumulation of sarcomeres in parallel (Correct Answer)
Explanation: ***Accumulation of sarcomeres in parallel***
- **Concentric left ventricular hypertrophy** (LVH) is primarily caused by **pressure overload**, such as from **hypertension**. The heart muscle cells respond to this increased afterload by adding new sarcomeres predominantly in parallel, leading to an increase in wall thickness with a relatively preserved chamber size.
- This structural adaptation helps the ventricle generate greater force to overcome the elevated systemic vascular resistance.
*Accumulation of protein fibrils*
- Accumulation of protein fibrils, specifically **amyloid protein**, is characteristic of **cardiac amyloidosis**, which typically causes **restrictive cardiomyopathy**.
- While amyloidosis can cause thickening of the ventricular walls, its clinical presentation (e.g., often with proteinuria, peripheral neuropathy) and echocardiographic findings (e.g., granular "sparkling" myocardium) differ from classic concentric LVH due to hypertension.
*Accumulation of glycogen*
- Excessive accumulation of glycogen within cardiac myocytes is a hallmark of **glycogen storage diseases**, such as **Pompe disease**.
- These conditions often lead to hypertrophic cardiomyopathy, but they are genetic disorders usually presenting earlier in life, and hypertension is not the underlying mechanism for the hypertrophy.
*Infiltration of T lymphocytes*
- Infiltration of T lymphocytes is a characteristic feature of **inflammatory myocarditis**, which can lead to various cardiac dysfunctions including hypertrophy, but the primary mechanism is inflammation and myocyte damage, not a direct response to pressure overload.
- The patient's history of **hypertension** and the specific finding of **concentric LVH** strongly point away from an inflammatory cause.
*Deposition of endomyocardial collagen*
- Deposition of endomyocardial collagen is associated with conditions like **restrictive cardiomyopathy**, where there is excessive fibrosis leading to stiffening of the ventricular walls and impaired diastolic filling.
- While hypertension can eventually lead to some degree of fibrosis, the initial and primary mechanism of concentric hypertrophy is the addition of sarcomeres due to increased pressure overload, rather than collagen deposition being the dominant cause of the hypertrophy itself.
Question 126: A 49-year-old woman presents to the office because of tremors for 2 months. She says that her hands have been shaking a lot, especially when she feels stressed. In addition, she has been sweating more than usual and has lost 8 kg (17.6 lb) in the last 2 months. She has a past medical history of vitiligo. Her vital signs are a heart rate of 98/min, a respiratory rate of 14/min, a temperature of 37.6°C (99.7°F), and a blood pressure of 115/75 mm Hg. Physical examination shows a fine, bilateral hand tremor and a diffuse goiter. Which of the following hormonal imbalances is most likely present?
A. Low TSH, high free T4, and high free T3 (Correct Answer)
B. High TSH, high free T4, and high free T3
C. High TSH, normal free T4, and normal free T3
D. Low TSH, normal free T4, and normal free T3
E. High TSH, low free T4, and low free T3
Explanation: ***Low TSH, high free T4, and high free T3***
- The patient's symptoms (tremors, sweating, weight loss, tachycardia, goiter) are classic for **hyperthyroidism**, which is typically characterized by **low TSH** due to negative feedback and **elevated free T4 and T3** levels.
- Her history of **vitiligo**, an autoimmune condition, further supports an autoimmune thyroid disorder like **Graves' disease**, a common cause of hyperthyroidism.
*High TSH, high free T4, and high free T3*
- This pattern would indicate **secondary hyperthyroidism**, caused by a TSH-secreting pituitary adenoma.
- While TSH would be high, it is a much rarer cause of hyperthyroidism compared to primary causes.
*High TSH, normal free T4, and normal free T3*
- This hormonal profile is characteristic of **subclinical hypothyroidism** or a **compensated primary hypothyroidism** early in its course.
- The patient's symptoms are inconsistent with hypothyroidism.
*Low TSH, normal free T4, and normal free T3*
- This suggests **subclinical hyperthyroidism**, where TSH is suppressed but thyroid hormone levels are still within the normal range.
- The patient's prominent and severe symptoms (tremors, significant weight loss, goiter) indicate overt hyperthyroidism, not subclinical disease.
*High TSH, low free T4, and low free T3*
- This is the classic hormonal profile for **primary hypothyroidism**, where the thyroid gland is failing to produce sufficient hormones, leading to elevated TSH.
- The patient's symptoms of nervousness, weight loss, and tremors are directly opposite to those seen in hypothyroidism.
Question 127: A previously healthy 57-year-old man comes to the emergency department because of acute retrosternal chest pain that radiates to his back. The pain started suddenly while he was having dinner. A few moments prior to the onset of the pain, he experienced discomfort when trying to eat or drink anything. On the way to the hospital he took a sublingual nitrate tablet that he had at home, which helped relieve the pain. His pulse is 80/min, respirations are 14/min, and blood pressure is 144/88 mm Hg. Examination shows no other abnormalities. An ECG shows a normal sinus rhythm with no ST-segment abnormalities. An esophagogram is done and shows areas of diffuse, uncoordinated spasms in several segments along the length of the esophagus. This patient's condition is most likely to show which of the following findings?
A. Esophageal manometry shows hypertensive contractions
B. Ultrasonography shows a mass at the gastroesophageal junction
C. Esophageal manometry shows simultaneous multi-peak contractions (Correct Answer)
D. Endoscopy shows multiple mucosal erosions
E. Serology shows elevated CK-MB levels
Explanation: ***Esophageal manometry shows simultaneous multi-peak contractions***
- The patient's symptoms of sudden retrosternal chest pain radiating to the back, discomfort with eating and drinking, and pain relief with nitrates suggest an **esophageal spasm**, which is confirmed by the esophagogram showing **diffuse, uncoordinated spasms**. **Simultaneous, multi-peak contractions** are characteristic manometric findings in **diffuse esophageal spasm**.
- Relief with **nitrates** is common in esophageal spasms because they relax smooth muscle.
*Esophageal manometry shows hypertensive contractions*
- **Hypertensive contractions** are characteristic of **nutcracker esophagus** (or hypercontractile esophagus), a condition marked by high-amplitude peristaltic contractions, which are distinct from the uncoordinated spasms seen in this patient.
- While both can cause chest pain, the esophagogram showing **diffuse, uncoordinated spasms** points away from nutcracker esophagus.
*Ultrasonography shows a mass at the gastroesophageal junction*
- A mass at the **gastroesophageal junction** would suggest a **tumor** or other structural lesion, which would typically cause progressive dysphagia and weight loss, not acute, intermittent pain relieved by nitrates.
- **Esophageal spasms** are functional disorders of motility, not structural abnormalities detectable by ultrasound at the GE junction.
*Endoscopy shows multiple mucosal erosions*
- **Mucosal erosions** are indicative of **gastroesophageal reflux disease (GERD)** or esophagitis, which would present with heartburn, regurgitation, and pain primarily related to acid exposure, not the acute, spasm-like pain described.
- Endoscopy is usually normal in **diffuse esophageal spasm**.
*Serology shows elevated CK-MB levels*
- **Elevated CK-MB levels** are a marker for **myocardial infarction (MI)**, which is an important differential for chest pain. However, the normal ECG and relief with nitrates (though nitrates can relieve both cardiac and esophageal pain) coupled with the esophagogram findings definitively rule out MI as the primary diagnosis.
- The patient's symptoms are fully explained by the **esophageal findings and history**.
Question 128: Nine healthy subjects participate in a study of gastric secretions. Subjects are asked to eat a meal at hour 0, at which time the pH of stomach contents and rate of stomach acid secretions are measured over the next 4 hours. Results of the study are shown. Which of the following mediators is most active at point A in the graph?
A. Secretin
B. Acetylcholine (Correct Answer)
C. Prostaglandin
D. Glucose-dependent insulinotropic peptide
E. Somatostatin
Explanation: ***Acetylcholine***
- At point A, the **pH is low** and the **rate of stomach acid secretion is high**, indicating a robust post-meal response for digestion.
- **Acetylcholine** is a primary neurocrine stimulant of gastric acid secretion, acting both directly on parietal cells and indirectly by stimulating histamine release, making it highly active during increased acid output.
*Secretin*
- **Secretin** primarily acts to *inhibit* gastric acid secretion and stimulate bicarbonate and fluid secretion from the pancreas and liver, particularly in response to low pH in the duodenum.
- It would be *less active* during a phase of high gastric acid secretion, as its main role is to neutralize acid after it leaves the stomach.
*Prostaglandin*
- **Prostaglandins (e.g., PGE2)** play a *cytoprotective role* in the stomach, inhibiting acid secretion and increasing mucus and bicarbonate production.
- Their activity would *reduce* the rate of stomach acid secretion, which is contrary to the high secretion rate observed at point A.
*Glucose-dependent insulinotropic peptide*
- **Glucose-dependent insulinotropic peptide (GIP)** primarily stimulates **insulin release** from pancreatic β-cells in response to ingested glucose and fat.
- While it has slight *inhibitory effects* on gastric acid secretion, its main role is metabolic, not directly related to the observed high acid secretion or low pH.
*Somatostatin*
- **Somatostatin** is a potent *inhibitor* of gastric acid secretion, acting directly on parietal cells and indirectly by inhibiting the release of histamine and gastrin.
- Therefore, it would be *less active* during a period of high gastric acid secretion, as its function is to suppress such activity.
Question 129: An investigator is studying cardiomyocytes in both normal and genetically modified mice. Both groups are observed after aerobic exercise and their heart rates are recorded and compared. After a 10-minute session on a treadmill, the average pulse measured in the normal mice is 680/min, whereas in the genetically modified mice it is only 160/min. Which of the following is most likely to account for the normal mice's ability to achieve higher heart rates during exercise stress?
A. Lower threshold potential for Ca2+ channel opening
B. Greater cardiomyocyte size
C. Lower number of gap junctions
D. Greater ratio of heart to body weight
E. Greater T-tubule density (Correct Answer)
Explanation: ***Greater T-tubule density***
- A **greater T-tubule density** in normal mice's cardiomyocytes enhances **calcium-induced calcium release**, allowing for more efficient and rapid muscle contraction and relaxation cycles, supporting higher heart rates.
- T-tubules facilitate the rapid propagation of the **action potential** deep into the myocyte, ensuring synchronized contraction.
*Lower threshold potential for Ca2+ channel opening*
- A lower threshold potential for Ca2+ channel opening would likely lead to increased contractility at baseline or earlier excitation, but it does not directly explain the ability to sustain a high heart rate during exercise.
- This effect might even lead to earlier fatigue or arrhythmias without proper regulation.
*Greater cardiomyocyte size*
- While larger cardiomyocytes might indicate hypertrophy, it generally implies **slower contraction and relaxation kinetics** due to increased diffusion distances and a larger volume to depolarize.
- This would typically lead to a **lower maximum heart rate** rather than a higher one during intense exercise.
*Lower number of gap junctions*
- A lower number of **gap junctions** would impair the efficient and rapid **electrical coupling** between cardiomyocytes.
- This would result in **slower conduction velocity** and a less coordinated, slower heart rate, making higher heart rates more difficult to achieve.
*Greater ratio of heart to body weight*
- A greater heart-to-body weight ratio could indicate a larger heart, which might be correlated with greater stroke volume, but doesn't specifically explain the cellular mechanism for achieving a **higher heart rate**.
- While larger hearts might be more efficient, the question focuses on the cellular ability to beat faster during stress, which relates more to excitation-contraction coupling.
Question 130: A 33-year-old female presents to her primary care physician complaining of heat intolerance and difficulty sleeping over a one month period. She also reports that she has lost 10 pounds despite no changes in her diet or exercise pattern. More recently, she has developed occasional unprovoked chest pain and palpitations. Physical examination reveals a nontender, mildly enlarged thyroid gland. Her patellar reflexes are 3+ bilaterally. Her temperature is 99°F (37.2°C), blood pressure is 135/85 mmHg, pulse is 105/min, and respirations are 18/min. Laboratory analysis is notable for decreased TSH. Which of the following pathophysiologic mechanisms contributed to the cardiovascular symptoms seen in this patient?
A. Increased numbers of α1-adrenergic receptors
B. Increased sensitivity of β1-adrenergic receptors (Correct Answer)
C. Decreased numbers of α2-adrenergic receptors
D. Decreased sensitivity of β2-adrenergic receptors
E. Decreased numbers of α1-adrenergic receptors
Explanation: ***Increased sensitivity of β1-adrenergic receptors***
- Elevated thyroid hormone levels in **hyperthyroidism** increase the expression and sensitivity of **β1-adrenergic receptors** in the heart.
- This heightened sensitivity leads to an exaggerated response to **catecholamines**, contributing to symptoms like **tachycardia**, **palpitations**, and **chest pain**.
*Increased numbers of α1-adrenergic receptors*
- While thyroid hormones can influence adrenergic receptor expression, the primary cardiovascular effects of hyperthyroidism are mediated by **β-adrenergic receptors**, not α1.
- An increase in α1-adrenergic receptors would primarily lead to **vasoconstriction**, which is not the predominant cardiovascular pathology in hyperthyroidism where **increased heart rate** and contractility are key.
*Decreased numbers of α1-adrenergic receptors*
- This would generally lead to **vasodilation** and possibly hypotension, which is contrary to the **palpitations** and **chest pain** seen in the patient's hyperthyroid state.
- Hyperthyroidism tends to increase cardiac output and contractility rather than decrease peripheral resistance through reduced α1 receptors.
*Decreased numbers of α2-adrenergic receptors*
- **Alpha-2 adrenergic receptors** are often involved in **negative feedback** to reduce sympathetic outflow from the central nervous system.
- A decrease in these receptors would theoretically increase sympathetic activity, but the direct cardiovascular effects in hyperthyroidism are primarily due to altered **β-adrenergic receptor** function.
*Decreased sensitivity of β2-adrenergic receptors*
- **Beta-2 adrenergic receptors** are primarily found in smooth muscle (e.g., bronchioles, blood vessels) and mediate **vasodilation and bronchodilation**.
- Decreased sensitivity would lead to **vasoconstriction** and **bronchoconstriction**, which are not characteristic cardiovascular or pulmonary findings in hyperthyroidism.
