Phases of the cardiac cycle — MCQs

10 questions

A 62-year-old man comes to the physician for decreased exercise tolerance. Over the past four months, he has noticed progressively worsening shortness of breath while walking his dog. He also becomes short of breath when lying in bed at night. His temperature is 36.4°C (97.5°F), pulse is 82/min, respirations are 19/min, and blood pressure is 155/53 mm Hg. Cardiac examination shows a high-pitch, decrescendo murmur that occurs immediately after S2 and is heard best along the left sternal border. There is an S3 gallop. Carotid pulses are strong. Which of the following is the most likely diagnosis?

A 76-year-old male with a history of chronic uncontrolled hypertension presents to the emergency room following an episode of syncope. He reports that he felt lightheaded and experienced chest pain while walking his dog earlier in the morning. He notes that he has experienced multiple similar episodes over the past year. A trans-esophageal echocardiogram demonstrates a thickened, calcified aortic valve with left ventricular hypertrophy. Which of the following heart sounds would likely be heard on auscultation of this patient?

A 64-year-old man presents to his physician for a scheduled follow-up visit. He has chronic left-sided heart failure with systolic dysfunction. His current regular medications include captopril and digoxin, which were started after his last episode of symptomatic heart failure approximately 3 months ago. His last episode of heart failure was accompanied by atrial fibrillation, which followed an alcohol binge over a weekend. Since then he stopped drinking. He reports that he has no current symptoms at rest and is able to perform regular physical exercise without limitation. On physical examination, mild bipedal edema is noted. The physician suggested to him that he should discontinue digoxin and continue captopril and scheduled him for the next follow-up visit. Which of the following statements best justifies the suggestion made by the physician?

A 61-year-old man comes to the physician because of a 3-month history of fatigue and progressively worsening shortness of breath that is worse when lying down. Recently, he started using two pillows to avoid waking up short of breath at night. Examination shows a heart murmur. A graph with the results of cardiac catheterization is shown. Given this patient's valvular condition, which of the following murmurs is most likely to be heard on cardiac auscultation?

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 64-year-old man presents to the emergency department because he has been experiencing increased shortness of breath for the last 2 weeks. Specifically, he says that he can barely walk up the stairs to his apartment before he feels winded. In addition, he has been waking up at night gasping for breath and has only been able to sleep propped up on 2 more pillows than usual. Physical exam reveals jugular venous distention as well as pitting lower extremity edema. Which of the following abnormal sounds will most likely be heard in this patient?

A 34-year-old male is brought to the emergency department. He has prior hospitalizations for opiate overdoses, but today presents with fever, chills, rigors and malaise. On physical exam vitals are temperature: 100.5 deg F (38.1 deg C), pulse is 105/min, blood pressure is 135/60 mmHg, and respirations are 22/min. You note the following findings on the patient's hands (Figures A and B). You note that as the patient is seated, his head bobs with each successive heart beat. Which of the following findings is most likely present in this patient?

An 80-year-old man presents to the emergency department because of gnawing substernal chest pain that started an hour ago and radiates to his neck and left jaw. A 12-lead ECG is obtained and shows ST-segment elevation with newly developing Q waves. He is admitted for treatment. 4 days after hospitalization he suddenly develops altered mental status, and his blood pressure falls from 115/75 mm Hg to 80/40 mm Hg. Physical examination shows jugular venous distention, pulsus paradoxus, and distant heart sounds. What is the most likely cause of this patient's condition?

An abnormal wave is noted on a routine ECG. The wave in question represents which of the following electrical events in the cardiac cycle?

Q10

A 12-year-old girl is brought to an oncologist, as she was recently diagnosed with a rare form of cancer. Cytogenetic studies reveal that the tumor is responsive to vinblastine, which is a cell-cycle specific anticancer agent. It acts on the M phase of the cell cycle and inhibits the growth of cells. Which of the following statements best describes the regulation of the cell cycle?

Phases of the cardiac cycle US Medical PG Practice Questions and MCQs

Question 1: A 62-year-old man comes to the physician for decreased exercise tolerance. Over the past four months, he has noticed progressively worsening shortness of breath while walking his dog. He also becomes short of breath when lying in bed at night. His temperature is 36.4°C (97.5°F), pulse is 82/min, respirations are 19/min, and blood pressure is 155/53 mm Hg. Cardiac examination shows a high-pitch, decrescendo murmur that occurs immediately after S2 and is heard best along the left sternal border. There is an S3 gallop. Carotid pulses are strong. Which of the following is the most likely diagnosis?

A. Aortic valve regurgitation (Correct Answer)
B. Tricuspid valve regurgitation
C. Mitral valve prolapse
D. Mitral valve regurgitation
E. Mitral valve stenosis

Explanation: ***Aortic valve regurgitation*** - A **high-pitch, decrescendo murmur immediately after S2** and heard best along the **left sternal border** is characteristic of **aortic regurgitation**. - Symptoms like **dyspnea on exertion** and **orthopnea**, an **S3 gallop**, and a **wide pulse pressure** (155/53 mmHg) further support heart failure due to chronic aortic regurgitation. *Tricuspid valve regurgitation* - This typically presents with a **holosystolic murmur** best heard at the **left lower sternal border** that increases with inspiration. - Clinical signs often include **jugular venous distension** and **peripheral edema**, not primarily a decrescendo diastolic murmur. *Mitral valve prolapse* - Characterized by a **mid-systolic click** followed by a **late systolic murmur**, and symptomatically may be asymptomatic or cause palpitations. - The described diastolic murmur and symptoms of heart failure do not align with mitral valve prolapse. *Mitral valve regurgitation* - Typically presents as a **holosystolic murmur** heard best at the **apex** and often radiating to the axilla. - While it can cause dyspnea and an S3, the character and timing of the murmur reported (decrescendo, immediately after S2) are inconsistent with mitral regurgitation. *Mitral valve stenosis* - This condition presents with a **diastolic rumble** heard best at the **apex** with an opening snap. - The murmur described is a high-pitch decrescendo murmur, which is distinct from the low-pitched rumble of mitral stenosis.

Question 2: A 76-year-old male with a history of chronic uncontrolled hypertension presents to the emergency room following an episode of syncope. He reports that he felt lightheaded and experienced chest pain while walking his dog earlier in the morning. He notes that he has experienced multiple similar episodes over the past year. A trans-esophageal echocardiogram demonstrates a thickened, calcified aortic valve with left ventricular hypertrophy. Which of the following heart sounds would likely be heard on auscultation of this patient?

A. Crescendo-decrescendo murmur radiating to the carotids that is loudest at the right upper sternal border (Correct Answer)
B. Diastolic rumble following an opening snap with an accentuated S1
C. Early diastolic high-pitched blowing decrescendo murmur that is loudest at the left sternal border
D. Holosystolic murmur radiating to the axilla that is loudest at the apex
E. Midsystolic click that is loudest at the apex

Explanation: ***Crescendo-decrescendo murmur radiating to the carotids that is loudest at the right upper sternal border*** - The patient's symptoms of **syncope**, **chest pain**, and findings of a **thickened, calcified aortic valve** with **left ventricular hypertrophy** are classic for **aortic stenosis**. - Aortic stenosis classically presents with a **systolic ejecting crescendo-decrescendo murmur** which is loudest at the **right upper sternal border**, and often **radiates to the carotids**. *Diastolic rumble following an opening snap with an accentuated S1* - This description is characteristic of **mitral stenosis**, which is typically caused by **rheumatic fever**. - Mitral stenosis would present with dyspnea and fatigue, unlike the syncope and chest pain seen in this patient. *Early diastolic high-pitched blowing decrescendo murmur that is loudest at the left sternal border* - This murmur describes **aortic regurgitation**, where blood flows back into the left ventricle during diastole. - While aortic regurgitation can cause heart failure symptoms, the echocardiogram shows a thickened, calcified valve more consistent with stenosis. *Midsystolic click that is most prominent that is loudest at the apex* - A **midsystolic click** followed by a **late systolic murmur** is characteristic of **mitral valve prolapse**. - Symptoms of mitral valve prolapse can include atypical chest pain and palpitations, but not generally exertional syncope or the severe structural changes seen in the aortic valve. *Holosystolic murmur radiating to the axilla that is loudest at the apex* - This is the classic description of **mitral regurgitation**, indicating blood flow back into the left atrium during systole. - Mitral regurgitation is associated with symptoms of heart failure and fatigue, but not usually the anginal chest pain and syncope in a patient with a calcified aortic valve.

Question 3: A 64-year-old man presents to his physician for a scheduled follow-up visit. He has chronic left-sided heart failure with systolic dysfunction. His current regular medications include captopril and digoxin, which were started after his last episode of symptomatic heart failure approximately 3 months ago. His last episode of heart failure was accompanied by atrial fibrillation, which followed an alcohol binge over a weekend. Since then he stopped drinking. He reports that he has no current symptoms at rest and is able to perform regular physical exercise without limitation. On physical examination, mild bipedal edema is noted. The physician suggested to him that he should discontinue digoxin and continue captopril and scheduled him for the next follow-up visit. Which of the following statements best justifies the suggestion made by the physician?

A. Long-term digoxin therapy produces significant survival benefits in patients with heart failure, but at the cost of increased heart failure-related admissions.
B. Both captopril and digoxin are likely to improve the long-term survival of the patient with heart failure, but digoxin has more severe side effects.
C. Captopril is likely to improve the long-term survival of the patient with heart failure, unlike digoxin.
D. Digoxin does not benefit patients with left-sided heart failure in the absence of atrial fibrillation.
E. Digoxin is useful to treat atrial fibrillation, but does not benefit patients with systolic dysfunction who are in sinus rhythm. (Correct Answer)

Explanation: ***Digoxin is useful to treat atrial fibrillation, but does not benefit patients with systolic dysfunction who are in sinus rhythm.*** - The patient's **atrial fibrillation** was likely triggered by the alcohol binge and has since resolved, suggesting he is now in **sinus rhythm**. - Digoxin's primary benefit in heart failure with **systolic dysfunction** (HFrEF) is to control ventricular rate in patients with **atrial fibrillation**, but it does not offer survival benefit in HFrEF patients who are in **sinus rhythm** and well-managed with other therapies. *Long-term digoxin therapy produces significant survival benefits in patients with heart failure, but at the cost of increased heart failure-related admissions.* - This statement is incorrect; digoxin has been shown to **reduce hospital admissions** for heart failure, but it does **not provide a significant survival benefit** in patients with HFrEF in sinus rhythm. - The main benefit of digoxin in HFrEF is to improve symptoms and quality of life, alongside reducing hospitalizations, but not prolonging life. *Both captopril and digoxin are likely to improve the long-term survival of the patient with heart failure, but digoxin has more severe side effects.* - **Captopril (an ACE inhibitor)** does improve **long-term survival** in heart failure, but **digoxin does not** demonstrably improve survival. - While digoxin can have side effects, its lack of survival benefit for HFrEF in sinus rhythm is the primary reason for discontinuation, not just side effect severity. *Captopril is likely to improve the long-term survival of the patient with heart failure, unlike digoxin.* - This statement is partially correct that **captopril improves survival**, but it does not fully explain the physician's decision to discontinue digoxin. - The key missing piece is the patient's current **sinus rhythm** and the lack of benefit of digoxin in that specific context for HFrEF. *Digoxin does not benefit patients with left-sided heart failure in the absence of atrial fibrillation.* - This statement is nearly correct, but "left-sided heart failure" is broad. It is specifically in patients with **systolic dysfunction (HFrEF)** who are in **sinus rhythm** that digoxin lacks significant benefit beyond symptom control, and does not provide survival benefit.

Question 4: A 61-year-old man comes to the physician because of a 3-month history of fatigue and progressively worsening shortness of breath that is worse when lying down. Recently, he started using two pillows to avoid waking up short of breath at night. Examination shows a heart murmur. A graph with the results of cardiac catheterization is shown. Given this patient's valvular condition, which of the following murmurs is most likely to be heard on cardiac auscultation?

A. High-frequency, diastolic murmur heard best at the 2nd left intercostal space
B. Harsh, late systolic murmur that radiates to the carotids
C. Blowing, early diastolic murmur heard best at the Erb point
D. High-pitched, holosystolic murmur that radiates to the axilla (Correct Answer)
E. Rumbling, delayed diastolic murmur heard best at the cardiac apex

Explanation: ***High-pitched, holosystolic murmur that radiates to the axilla*** - The patient's symptoms of **fatigue**, **dyspnea on exertion** and **orthopnea**, combined with a heart murmur, are highly suggestive of **heart failure** caused by **mitral regurgitation**. - A **high-pitched**, **holosystolic murmur** heard best at the **apex** and **radiating to the axilla** is the classic description of mitral regurgitation. *High-frequency, diastolic murmur heard best at the 2nd left intercostal space* - This describes the murmur of **pulmonary regurgitation**, which is typically heard best at the **left upper sternal border**. - The patient's symptoms are more consistent with left-sided heart failure due to a different valvular issue. *Harsh, late systolic murmur that radiates to the carotids* - This is the characteristic murmur of **aortic stenosis**, which is heard best at the **right upper sternal border**. - While aortic stenosis can cause similar symptoms, the description of the murmur and the specific context of heart failure symptoms here point away from it. *Blowing, early diastolic murmur heard best at the Erb point* - This describes the **diastolic murmur of aortic regurgitation**, often heard best at the **Erb's point** (3rd intercostal space, left sternal border). - While aortic regurgitation can cause heart failure, its murmur is diastolic, not holosystolic, and the maximal intensity and radiation differ from the classic mitral regurgitation. *Rumbling, delayed diastolic murmur heard best at the cardiac apex* - This is the characteristic murmur of **mitral stenosis**, which is typically preceded by an **opening snap**. - Mitral stenosis would lead to different hemodynamic changes and often presents with symptoms related to pulmonary congestion, but the murmur timing and quality are distinct from a holosystolic murmur of regurgitation.

Question 5: 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

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.

Question 6: A 64-year-old man presents to the emergency department because he has been experiencing increased shortness of breath for the last 2 weeks. Specifically, he says that he can barely walk up the stairs to his apartment before he feels winded. In addition, he has been waking up at night gasping for breath and has only been able to sleep propped up on 2 more pillows than usual. Physical exam reveals jugular venous distention as well as pitting lower extremity edema. Which of the following abnormal sounds will most likely be heard in this patient?

A. Opening snap
B. Extra heart sound in early diastole (Correct Answer)
C. Extra heart sound in late diastole
D. Fixed splitting
E. Parasternal holosystolic murmur

Explanation: ***Extra heart sound in early diastole*** - The patient's symptoms (dyspnea, orthopnea, paroxysmal nocturnal dyspnea, jugular venous distention, edema) are classic for **congestive heart failure (CHF)**. - An **S3 gallop** is an extra heart sound occurring in **early diastole** and is pathognomonic for **volume overload** and **ventricular dysfunction** in CHF. *Opening snap* - An **opening snap** is typically heard in **mitral stenosis**, which is not directly indicated by the patient's symptoms of volume overload from heart failure. - This sound occurs shortly after S2 as the stenotic mitral valve opens. *Extra heart sound in late diastole* - An extra heart sound in **late diastole** is often an **S4 gallop**, which indicates **poor ventricular compliance** (e.g., in hypertension or aortic stenosis) rather than the pronounced volume overload suggested by the current presentation. - An S4 is heard just before S1, as the atria contract to push blood into a stiff ventricle. *Fixed splitting* - **Fixed splitting of S2** is characteristic of an **atrial septal defect (ASD)**, where there is a constant delay in pulmonic valve closure, independent of respiration. - This is not a typical finding in the context of acute decompensated heart failure as described. *Parasternal holosystolic murmur* - A **parasternal holosystolic murmur** is typically associated with **ventricular septal defect (VSD)** or **tricuspid regurgitation**. - While tricuspid regurgitation can occur secondary to right heart failure, the most immediate and common auscultatory finding for overall heart failure with volume overload is an S3.

Question 7: A 34-year-old male is brought to the emergency department. He has prior hospitalizations for opiate overdoses, but today presents with fever, chills, rigors and malaise. On physical exam vitals are temperature: 100.5 deg F (38.1 deg C), pulse is 105/min, blood pressure is 135/60 mmHg, and respirations are 22/min. You note the following findings on the patient's hands (Figures A and B). You note that as the patient is seated, his head bobs with each successive heart beat. Which of the following findings is most likely present in this patient?

A. A holosystolic murmur at the 4th intercostal midclavicular line
B. A water-hammer pulse when palpating the radial artery (Correct Answer)
C. Decreased blood pressure as measured in the lower extremities compared to the upper extremities
D. A harsh crescendo-decrescendo systolic murmur in the right second intercostal space
E. A consistent gallop with an S4 component

Explanation: ***A water-hammer pulse when palpating the radial artery*** - The patient's history of **opiate overdose**, fever, chills, and the presence of **Janeway lesions** (Figures A and B) on the hands strongly suggest **infective endocarditis**. The head bobbing (Musset's sign) indicates **severe aortic regurgitation**. - **Water-hammer pulse** (Corrigan's pulse) is a classic sign of **severe aortic regurgitation**, characterized by a rapid, forceful arterial pulse that quickly collapses due to a large stroke volume and rapid diastolic runoff. *A holosystolic murmur at the 4th intercostal midclavicular line* - A holosystolic murmur at the 4th intercostal midclavicular line is typically associated with **mitral regurgitation**, which is less likely given the prominent signs of aortic regurgitation. - While endocarditis can affect the mitral valve, the specific clinical signs point towards **aortic valve involvement**. *Decreased blood pressure as measured in the lower extremities compared to the upper extremities* - This finding is characteristic of **coarctation of the aorta**, a congenital heart defect, which is not suggested by the patient's presentation or risk factors. - The patient's symptoms are more consistent with an acute infectious process affecting the heart valves. *A harsh crescendo-decrescendo systolic murmur in the right second intercostal space* - A harsh crescendo-decrescendo systolic murmur in the right second intercostal space is typical of **aortic stenosis**. - While aortic insufficiency is present, the murmur for uncomplicated aortic insufficiency is usually a **diastolic decrescendo murmur**, not a harsh systolic murmur. *A consistent gallop with an S4 component* - An S4 gallop is typically heard in conditions involving **decreased ventricular compliance** (e.g., severe hypertension, aortic stenosis, hypertrophic cardiomyopathy). - While endocarditis can cause heart failure, an S4 gallop is not a direct or primary sign of **aortic regurgitation**. An S3 gallop is more commonly associated with **volume overload** and heart failure, which might develop in severe aortic regurgitation.

Question 8: An 80-year-old man presents to the emergency department because of gnawing substernal chest pain that started an hour ago and radiates to his neck and left jaw. A 12-lead ECG is obtained and shows ST-segment elevation with newly developing Q waves. He is admitted for treatment. 4 days after hospitalization he suddenly develops altered mental status, and his blood pressure falls from 115/75 mm Hg to 80/40 mm Hg. Physical examination shows jugular venous distention, pulsus paradoxus, and distant heart sounds. What is the most likely cause of this patient's condition?

A. Pericardial inflammation
B. Compression of heart chambers by blood in the pericardial space (Correct Answer)
C. Arrhythmia caused by ventricular fibrillation
D. Rupture of papillary muscle
E. Acute pulmonary edema from left heart failure

Explanation: ***Compression of heart chambers by blood in the pericardial space*** - The patient's initial presentation with ST-elevation myocardial infarction (STEMI) and subsequent development of **hypotension**, **jugular venous distention**, **pulsus paradoxus**, and **distant heart sounds** (Beck's triad) is highly indicative of **cardiac tamponade.** - In the context of a recent MI, this constellation of symptoms strongly suggests a **cardiac free wall rupture**, leading to blood accumulation in the pericardial sac and compression of the heart. - Free wall rupture typically occurs **3-7 days post-MI** and is a life-threatening mechanical complication. *Pericardial inflammation* - While pericardial inflammation (pericarditis) can occur post-MI, it typically manifests with **pleuritic chest pain** that is relieved by leaning forward and is often associated with a **pericardial friction rub.** - It does not typically lead to acute, severe hypotension, pulsus paradoxus, or sudden circulatory collapse in this manner without significant effusion and tamponade physiology. *Arrhythmia caused by ventricular fibrillation* - **Ventricular fibrillation** would cause immediate cardiac arrest and loss of consciousness, not a gradual development of hypotension, JVD, and pulsus paradoxus. - While arrhythmias are common post-MI, the specific physical findings point away from isolated VFib as the primary cause of hemodynamic collapse. *Acute pulmonary edema from left heart failure* - **Acute pulmonary edema** is a manifestation of **left heart failure**, characterized by severe dyspnea, orthopnea, and crackles on lung auscultation. - While left heart failure can cause hypotension in cardiogenic shock, it would not typically present with the classic signs of cardiac tamponade such as pulsus paradoxus, distant heart sounds, and prominent JVD without pulmonary congestion findings. *Rupture of papillary muscle* - **Papillary muscle rupture** leads to severe **acute mitral regurgitation**, causing acute pulmonary edema, a new holosystolic murmur, and often cardiogenic shock. - While it can lead to hypotension, it doesn't typically present with the classic signs of cardiac tamponade such as pulsus paradoxus and distant heart sounds; instead, a loud murmur would be prominent.

Question 9: An abnormal wave is noted on a routine ECG. The wave in question represents which of the following electrical events in the cardiac cycle?

A. Period between ventricular depolarization and repolarization
B. Atrial repolarization
C. Ventricular repolarization (Correct Answer)
D. Ventricular depolarization
E. Atrial depolarization

Explanation: ***Ventricular repolarization*** - The **T wave** represents ventricular repolarization, which is the electrical recovery phase of the ventricles after contraction - T wave abnormalities are among the most common ECG findings and include **T wave inversions** (myocardial ischemia, ventricular hypertrophy), **peaked T waves** (hyperkalemia), **flattened T waves** (hypokalemia, ischemia), and **biphasic T waves** - The T wave corresponds to **phase 3** of the ventricular action potential when potassium channels open and the membrane repolarizes *Period between ventricular depolarization and repolarization* - This describes the **ST segment**, which represents the period when ventricles are completely depolarized before repolarization begins - The **QT interval** encompasses both ventricular depolarization and repolarization (QRS + ST segment + T wave) - These are intervals or segments, not waves *Atrial repolarization* - Atrial repolarization occurs during ventricular depolarization and is represented by the **Ta wave** - This wave is typically **not visible** on standard ECG because it is **masked by the much larger QRS complex** and has very low amplitude - It cannot be identified as a distinct wave on routine ECGs *Ventricular depolarization* - The **QRS complex** represents ventricular depolarization, the electrical activation that triggers ventricular contraction - Normal QRS duration is **0.06-0.10 seconds** (3 small boxes or less) - QRS abnormalities include bundle branch blocks, ventricular hypertrophy patterns, and pre-excitation *Atrial depolarization* - The **P wave** represents atrial depolarization, the electrical activation that triggers atrial contraction - Normal P wave characteristics: **upright in leads I, II, aVF**; duration less than 0.12 seconds; amplitude less than 2.5 mm - P wave abnormalities include left atrial enlargement (broad, notched P waves) and right atrial enlargement (tall, peaked P waves)

Question 10: A 12-year-old girl is brought to an oncologist, as she was recently diagnosed with a rare form of cancer. Cytogenetic studies reveal that the tumor is responsive to vinblastine, which is a cell-cycle specific anticancer agent. It acts on the M phase of the cell cycle and inhibits the growth of cells. Which of the following statements best describes the regulation of the cell cycle?

A. Inhibitors of DNA synthesis act in the M phase of the cell cycle.
B. The G0 phase is the checkpoint before G1.
C. Cyclin-dependent activation of CDK1 (CDC2) takes place upon the entry of a cell into M phase of the cell cycle. (Correct Answer)
D. EGF from a blood clot stimulates the growth and proliferation of cells in the healing process.
E. Replication of the genome occurs in the M phase of the cell cycle.

Explanation: ***Cyclin-dependent activation of CDK1 (CDC2) takes place upon the entry of a cell into M phase of the cell cycle.*** - The **M-phase promoting factor (MPF)**, composed of **CDK1 (CDC2)** and **cyclin B**, is activated at the G2/M transition, driving the cell into mitosis. - Activation of CDK1 by **cyclin B binding** and subsequent dephosphorylation of threonine 161 is crucial for initiation of mitosis. *Inhibitors of DNA synthesis act in the M phase of the cell cycle.* - **Inhibitors of DNA synthesis**, such as **hydroxyurea** and **methotrexate**, primarily act during the **S phase** of the cell cycle, when DNA replication occurs. - The M phase is characterized by **mitosis** (nuclear division) and **cytokinesis** (cytoplasmic division), not DNA synthesis. *The G0 phase is the checkpoint before G1.* - The **G0 phase** is a **resting state** where cells exit the cell cycle and cease to divide, not a checkpoint before G1. - The main checkpoint before G1 is typically referred to as the **restriction point** or **G1 checkpoint**, which determines if a cell will commit to division. *EGF from a blood clot stimulates the growth and proliferation of cells in the healing process.* - While **EGF (Epidermal Growth Factor)** does stimulate cell growth and proliferation in healing, it is not primarily associated with blood clots. - **Platelets** in blood clots release growth factors like **PDGF (Platelet-Derived Growth Factor)** and **TGF-β (Transforming Growth Factor-beta)**, which are critical for wound healing. *Replication of the genome occurs in the M phase of the cell cycle.* - **Replication of the genome** (DNA synthesis) occurs during the **S phase** (synthesis phase) of the cell cycle. - The **M phase** is dedicated to **mitosis** (separation of duplicated chromosomes) and **cytokinesis**, where the cell divides into two daughter cells.

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