Pathophysiology Quiz: Myocardial Infarction Pathophysiology
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Myocardial Infarction PathophysiologyQuestion 1 of 20

Approximately 7 to 10 days after a myocardial infarction, the infarcted tissue is maximally soft and fragile. What two cellular processes are concurrently peaking during this period to create this state?

Phagocytosis by macrophages and the formation of new granulation tissue.
Neutrophil infiltration and coagulative necrosis.
Deposition of dense collagen and apoptosis of fibroblasts.
Interstitial edema and wavy fiber formation.
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Pathophysiology Quiz

Pathophysiology Quiz: Myocardial Infarction Pathophysiology

Practice Myocardial Infarction Pathophysiology in Pathophysiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Myocardial Infarction Pathophysiology, giving you a quick way to practice the rules, question types, and explanations that matter most for Pathophysiology.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

Approximately 7 to 10 days after a myocardial infarction, the infarcted tissue is maximally soft and fragile. What two cellular processes are concurrently peaking during this period to create this state?

  1. Phagocytosis by macrophages and the formation of new granulation tissue. (correct answer)
  2. Neutrophil infiltration and coagulative necrosis.
  3. Deposition of dense collagen and apoptosis of fibroblasts.
  4. Interstitial edema and wavy fiber formation.
Explanation: When you encounter questions about myocardial infarction healing, think about the timeline of tissue repair and the specific cellular events occurring at each stage. The 7-10 day period represents a critical transition phase in cardiac healing. At this timepoint, two key processes create maximum tissue vulnerability. First, macrophages are actively phagocytosing (eating) the dead cardiac muscle tissue from the initial infarct. This cleanup process literally removes structural tissue, creating gaps and weakening the myocardial wall. Simultaneously, new granulation tissue is forming - this consists of new capillaries, fibroblasts, and loose connective tissue that will eventually mature into scar tissue. However, this early granulation tissue is inherently fragile and lacks the structural integrity of normal myocardium or mature scar tissue. Answer A correctly identifies both processes that peak during this vulnerable period. Answer B describes events from the first few days post-MI - neutrophils dominate the initial inflammatory response (days 1-3), while coagulative necrosis occurs immediately after vessel occlusion. Answer C describes much later events - dense collagen deposition happens weeks later during scar maturation, not at 7-10 days when collagen is still loose and disorganized. Answer D refers to very early changes - interstitial edema and wavy fiber formation occur within hours to days of infarction. Remember that cardiac rupture risk peaks at 7-10 days precisely because of this combination of tissue removal and immature replacement tissue. Focus on understanding the temporal sequence of healing phases when studying MI pathophysiology.

Question 2

Three weeks after an extensive MI, a patient presents with fever, malaise, and sharp, pleuritic chest pain. A diagnosis of Dressler syndrome is made. The underlying pathophysiology of this condition is best described as:

  1. A delayed autoimmune reaction to cardiac antigens released from damaged myocytes. (correct answer)
  2. A direct bacterial infection of the necrotic myocardium spreading to the pericardium.
  3. Direct irritation of the pericardium by the epicardial surface of the fresh infarct.
  4. Formation of a ventricular aneurysm that mechanically irritates the overlying pericardial sac.
Explanation: When you encounter post-MI complications with a delayed onset (weeks after the initial event), think about autoimmune versus mechanical causes. The timing and symptomatology are crucial clues. Dressler syndrome represents a classic example of molecular mimicry and autoimmune pathophysiology. During the massive myocardial infarction, extensive myocyte death releases intracellular cardiac antigens that are normally hidden from the immune system. Over the following weeks, the immune system develops antibodies against these self-antigens, but these antibodies cross-react with similar antigens in the pericardium, pleura, and lung tissue. This delayed autoimmune reaction (typically 2-10 weeks post-MI) causes the characteristic triad of fever, pleuritic chest pain, and pericarditis. Answer A correctly describes this pathophysiology. Answer B is incorrect because Dressler syndrome is sterile—there's no bacterial infection involved. The inflammation is purely autoimmune. Answer C misses the mark because direct irritation would occur immediately after the MI, not weeks later, and the "fresh infarct" would be healing tissue by three weeks. Answer D describes a mechanical complication (ventricular aneurysm) that could cause pericardial irritation, but this doesn't explain the systemic symptoms like fever and malaise, nor the typical delayed presentation. Remember this pattern: immediate post-MI complications are usually mechanical (rupture, tamponade), while delayed complications (2+ weeks) often involve autoimmune mechanisms. The timing in the question stem is your biggest clue to distinguish between these pathways.

Question 3

A forensic pathologist examining heart tissue from an individual who died suddenly notes that the myocardial fibers in one region are elongated and wavy. There is early coagulation necrosis but a conspicuous absence of any neutrophilic infiltrate. Based on these specific microscopic findings, the most likely time from MI onset to death is:

  1. Less than 4 hours.
  2. 3 to 5 days.
  3. 24 to 36 hours.
  4. 4 to 12 hours. (correct answer)
Explanation: When you encounter questions about myocardial infarction timing, you need to recognize the characteristic histological changes that occur in a predictable sequence after coronary artery occlusion. The key findings described here—elongated wavy fibers, early coagulation necrosis, and absence of neutrophilic infiltrate—point to a specific timeframe in MI evolution. Wavy fibers result from passive stretching of dead myocytes by adjacent contracting viable muscle. Coagulation necrosis represents irreversible cell death with protein denaturation. The critical clue is the absence of neutrophils, which helps narrow the timing window. Here's why answer D (4 to 12 hours) is correct: This represents the early irreversible phase where structural changes are visible but inflammatory response hasn't begun. Wavy fibers appear within 4-12 hours, coagulation necrosis becomes evident around 4-6 hours, but neutrophilic infiltration doesn't start until 6-24 hours post-MI. Answer A (less than 4 hours) is too early—you wouldn't see clear coagulation necrosis yet, mainly just wavy fibers. Answer C (24-36 hours) is incorrect because neutrophils would be prominently present by this time as the acute inflammatory response peaks. Answer B (3-5 days) represents the stage where neutrophils are being replaced by macrophages and granulation tissue formation begins. Remember this sequence: wavy fibers first (4-12 hours), then neutrophils arrive (6-24 hours), followed by macrophages and healing (days to weeks). The absence of expected inflammatory cells is often as diagnostically important as their presence.

Question 4

A patient with multivessel coronary artery disease and chronic angina has a severely depressed left ventricular ejection fraction. After undergoing coronary artery bypass surgery, his ventricular function gradually improves over the following two months. This recovery of function is best attributed to the revascularization of:

  1. Stunned myocardium, which suffers from short-term dysfunction after acute ischemia.
  2. Hibernating myocardium, which is in a state of chronic contractile dysfunction due to persistent hypoperfusion. (correct answer)
  3. Infarcted myocardium, where necrotic tissue is replaced by functional myocytes.
  4. Remodeled myocardium, where hypertrophied cells have returned to their normal size.
Explanation: The correct answer is B. Hibernating myocardium refers to viable but chronically dysfunctional myocardial tissue resulting from long-term, reduced blood flow. It represents an adaptive state where the cells down-regulate their metabolic activity to survive. Revascularization restores blood flow, allowing these cells to gradually recover their contractile function over weeks to months. Stunned myocardium (A) describes acute dysfunction after reperfusion that recovers more quickly (hours to days).

Question 5

Which of the following sequences correctly represents the chronological order of the predominant inflammatory cell type and subsequent tissue changes during the first two weeks of healing after a myocardial infarction?

  1. Neutrophils -> Macrophages -> Granulation tissue -> Collagen scar (correct answer)
  2. Macrophages -> Neutrophils -> Granulation tissue -> Collagen scar
  3. Neutrophils -> Granulation tissue -> Macrophages -> Collagen scar
  4. Macrophages -> Granulation tissue -> Neutrophils -> Collagen scar
Explanation: When approaching questions about myocardial infarction healing, think about the body's systematic inflammatory response to tissue death. The healing process follows a predictable timeline driven by the type of damage and the body's need to clear debris before rebuilding. The correct sequence begins with neutrophils as the first responders within hours of the MI. These cells arrive quickly to begin the inflammatory response and start clearing cellular debris. After 2-3 days, macrophages take over as the dominant cell type, continuing the cleanup process more efficiently and releasing growth factors. Around day 4-7, granulation tissue forms as new blood vessels and fibroblasts begin organizing the repair scaffolding. Finally, collagen deposition peaks around week 2, creating the mature fibrous scar that replaces the dead myocardium. Choice B incorrectly places macrophages before neutrophils, but neutrophils are always the first inflammatory cells to arrive at injury sites. Choice C suggests granulation tissue forms before macrophages arrive, but you need macrophage-mediated cleanup and growth factor release before tissue regeneration can begin. Choice D makes the same macrophage timing error as B, plus incorrectly places neutrophils after granulation tissue formation. Study tip: Remember the mnemonic "Never Make Grilled Cheese" for the MI healing sequence: Neutrophils → Macrophages → Granulation tissue → Collagen scar. This pattern reflects the general principle that acute inflammation (neutrophils) precedes chronic inflammation (macrophages), which precedes tissue repair and remodeling.

Question 6

During an autopsy performed 5 days after a fatal myocardial infarction, the pathologist notes that the center of the infarct is markedly soft and yellow-tan, with a distinct hyperemic border. This gross appearance is primarily the result of:

  1. Deposition of new collagen and fibroblasts, which gives the tissue a pale, firm appearance.
  2. Hemorrhage and edema from ruptured capillaries, causing a dark red, mottled appearance.
  3. The onset of coagulation necrosis before the arrival of inflammatory cells.
  4. Enzymatic digestion of necrotic tissue by neutrophils and subsequent macrophage infiltration. (correct answer)
Explanation: The correct answer is D. The period from 3 to 7 days post-MI is characterized by central softening and a yellow-tan appearance. This is due to the robust inflammatory response. Neutrophils release proteolytic enzymes that begin to liquefy the necrotic debris, and macrophages arrive to phagocytose the dead cells and cellular remnants. The hyperemic border represents the ingrowth of new blood vessels (part of granulation tissue formation).

Question 7

A patient presents to the hospital 48 hours after the onset of severe chest pain. Blood tests show a markedly elevated cardiac troponin I (cTnI) level, while the creatine kinase-MB (CK-MB) level is elevated but already declining from its peak. This pattern is best explained by:

  1. A recent re-infarction event that has caused a second, smaller rise in CK-MB.
  2. The differing pharmacokinetics, with CK-MB being cleared from the circulation much faster than cTnI. (correct answer)
  3. The different cellular locations and release mechanisms of the two biomarkers from necrotic myocytes.
  4. Concomitant renal failure, which impairs troponin clearance more significantly than CK-MB clearance.
Explanation: The correct answer is B. CK-MB is a cytoplasmic enzyme that peaks around 12-24 hours and returns to normal by 48-72 hours due to rapid clearance. Cardiac troponin I, while also released early, remains elevated for 7-10 days because of slower clearance kinetics and continued release from the contractile apparatus breakdown. At 48 hours, this explains why CK-MB is declining while troponin remains markedly elevated.

Question 8

An experimental model of myocardial ischemia demonstrates that the 'point of no return,' or irreversible cell injury, is reached after approximately 30-40 minutes of occlusion. Which of the following ultrastructural findings is the most reliable indicator that a cardiomyocyte has crossed this threshold?

  1. Depletion of cytoplasmic glycogen stores.
  2. Swelling of mitochondria and the sarcoplasmic reticulum.
  3. Formation of large, amorphous densities within the mitochondrial matrix. (correct answer)
  4. Disaggregation of polysomes and clumping of nuclear chromatin.
Explanation: The correct answer is C. The formation of large, flocculent, amorphous densities within the mitochondrial matrix is a classic and definitive sign of irreversible mitochondrial damage and cell death. The other options describe changes that occur during the reversible phase of ischemic injury. Glycogen depletion (A), cellular and organelle swelling (B), and chromatin clumping (D) can all be reversed if blood flow is restored in time.

Question 9

In cases of subendocardial (NSTEMI) infarction resulting from severe coronary stenosis without total occlusion, the subendocardium is disproportionately affected compared to the epicardium. The primary reason for this heightened vulnerability is that the subendocardium:

  1. has a lower density of capillaries and relies more on anaerobic metabolism at baseline.
  2. is exposed to the highest intramyocardial pressure during systole, which impedes coronary blood flow. (correct answer)
  3. lacks the ability to develop collateral circulation, unlike the well-perfused epicardial layers.
  4. contains myocytes with fewer mitochondria, making them more susceptible to ATP depletion.
Explanation: The correct answer is B. The subendocardium is the most vulnerable region of the myocardium to ischemia for two main reasons. First, it is the last area to receive blood from the coronary arteries that penetrate from the epicardial surface. Second, and more importantly, it is subjected to the highest intramyocardial pressure during systolic contraction. This pressure physically compresses the small coronary vessels, limiting blood flow, a phenomenon that is exacerbated in the presence of a proximal stenosis.

Question 10

The 'border zone' surrounding a myocardial infarct scar is a known substrate for life-threatening ventricular arrhythmias. The primary electrophysiological abnormality in this region that promotes arrhythmogenesis is:

  1. Uniformly prolonged action potential duration in all surviving myocytes.
  2. The presence of hyper-excitable, spontaneously depolarizing fibroblasts.
  3. A heterogeneous mixture of viable myocytes, scar tissue, and ischemic cells creating slow, non-uniform conduction. (correct answer)
  4. Complete electrical silence due to high local potassium concentrations, which isolates the scar.
Explanation: The correct answer is C. The arrhythmogenic nature of the infarct border zone stems from its profound electrophysiological heterogeneity. It consists of intertwined bundles of surviving but often ischemic myocytes and non-conductive scar tissue. This creates a maze-like environment where electrical impulses can travel slowly and in a non-uniform, circuitous manner. This slow, fragmented conduction is the perfect substrate for re-entrant circuits, which are the mechanism for most post-MI ventricular tachycardias.

Question 11

The late phase of ventricular remodeling after a large MI involves changes in the non-infarcted, remote myocardium. Which of the following best describes the primary maladaptive process occurring in this remote tissue?

  1. Progressive infiltration by neutrophils leading to chronic, low-grade inflammation and fibrosis.
  2. Replacement of viable myocytes with adipose tissue, leading to a fatty infiltrate.
  3. Formation of hibernating myocardium as a protective mechanism to reduce oxygen demand.
  4. Pathological hypertrophy of surviving myocytes in response to increased hemodynamic load and neurohormonal stimulation. (correct answer)
Explanation: When you encounter questions about post-MI ventricular remodeling, focus on understanding how the heart adapts to losing functional myocardium. After a large MI, the surviving heart muscle faces dramatically increased workload, triggering compensatory mechanisms that eventually become maladaptive. The correct answer is D because remote, non-infarcted myocardium undergoes pathological hypertrophy as the primary late remodeling response. When significant myocardium is lost to infarction, surviving myocytes must generate more force to maintain cardiac output. This increased mechanical stress, combined with neurohormonal activation (particularly angiotensin II, norepinephrine, and aldosterone), stimulates myocyte hypertrophy. Initially compensatory, this hypertrophy becomes pathological over time, characterized by altered gene expression, impaired contractility, and increased susceptibility to arrhythmias and heart failure. Choice A is incorrect because neutrophil infiltration occurs primarily during the acute inflammatory phase (first few days) in the infarct zone itself, not as a chronic process in remote myocardium. Choice B misrepresents the remodeling process—while fatty infiltration can occur in the infarct scar, remote viable myocardium doesn't undergo replacement with adipose tissue. Choice C describes hibernating myocardium, which refers to chronically hypoperfused but viable tissue that reduces contractility to match reduced oxygen supply—this isn't the primary remodeling mechanism in well-perfused remote zones. Remember: Post-MI remodeling questions often test whether you understand the difference between acute healing responses (inflammation, scar formation) and chronic adaptive changes (hypertrophy, chamber dilation) in different regions of the heart.

Question 12

An experimental model of myocardial ischemia demonstrates that the 'point of no return,' or irreversible cell injury, is reached after approximately 30-40 minutes of occlusion. Which of the following ultrastructural findings is the most reliable indicator that a cardiomyocyte has crossed this threshold?

  1. Depletion of cytoplasmic glycogen stores.
  2. Swelling of mitochondria and the sarcoplasmic reticulum.
  3. Formation of large, amorphous densities within the mitochondrial matrix. (correct answer)
  4. Disaggregation of polysomes and clumping of nuclear chromatin.
Explanation: The correct answer is C. The formation of large, flocculent, amorphous densities within the mitochondrial matrix is a classic and definitive sign of irreversible mitochondrial damage and cell death. The other options describe changes that occur during the reversible phase of ischemic injury. Glycogen depletion (A), cellular and organelle swelling (B), and chromatin clumping (D) can all be reversed if blood flow is restored in time.

Question 13

An autopsy performed on a patient 12 days after a myocardial infarction is likely to show which of the following features at the center of the infarct zone on microscopic examination?

  1. A dense, hypocellular collagen scar with no remaining inflammation.
  2. Coagulation necrosis with a heavy infiltrate of intact neutrophils.
  3. Well-established granulation tissue with neovascularization and proliferating fibroblasts. (correct answer)
  4. A mix of wavy fibers and edematous stroma without significant cellular infiltration.
Explanation: The correct answer is C. The time point of 12 days falls within the 10-14 day window of healing. During this phase, the inflammatory cleanup is largely complete, and the repair process is well underway. Histologically, this is characterized by well-established granulation tissue, which is rich in new capillaries (neovascularization), fibroblasts actively synthesizing extracellular matrix, and a dwindling number of macrophages.

Question 14

The 'border zone' surrounding a myocardial infarct scar is a known substrate for life-threatening ventricular arrhythmias. The primary electrophysiological abnormality in this region that promotes arrhythmogenesis is:

  1. Uniformly prolonged action potential duration in all surviving myocytes.
  2. The presence of hyper-excitable, spontaneously depolarizing fibroblasts.
  3. A heterogeneous mixture of viable myocytes, scar tissue, and ischemic cells creating slow, non-uniform conduction. (correct answer)
  4. Complete electrical silence due to high local potassium concentrations, which isolates the scar.
Explanation: The correct answer is C. The arrhythmogenic nature of the infarct border zone stems from its profound electrophysiological heterogeneity. It consists of intertwined bundles of surviving but often ischemic myocytes and non-conductive scar tissue. This creates a maze-like environment where electrical impulses can travel slowly and in a non-uniform, circuitous manner. This slow, fragmented conduction is the perfect substrate for re-entrant circuits, which are the mechanism for most post-MI ventricular tachycardias.

Question 15

Myocardial free wall rupture is a catastrophic complication of MI that typically occurs 3-7 days after the initial event. The peak vulnerability during this specific time frame is a direct consequence of:

  1. The intense neutrophilic infiltrate, which has not yet been cleared from the infarct zone.
  2. The removal of necrotic myocardium by macrophages before significant collagen deposition has occurred. (correct answer)
  3. The formation of a rigid, non-compliant scar that creates high stress at the border zone.
  4. The peak of interstitial edema, which separates myocardial fibers and weakens the tissue.
Explanation: The correct answer is B. The period of 3-7 days post-MI is when the infarct is at its weakest. This is because the initial coagulation necrosis has occurred, and macrophages are now actively phagocytosing the dead tissue and cellular debris. This 'cleanup' phase results in significant softening and thinning of the myocardial wall. Granulation tissue, which will eventually provide structural integrity, has not yet sufficiently formed, creating a window of maximal vulnerability to rupture.

Question 16

A patient with a history of chronic stable angina develops a smaller-than-expected myocardial infarction following an acute coronary occlusion compared to another patient with no prior cardiac history who suffers the same occlusion. This protective effect is most likely mediated by:

  1. Myocardial hypertrophy developed in response to chronic pressure overload.
  2. Enhanced expression of heat shock proteins providing cellular ischemic preconditioning.
  3. Development of a robust network of coronary collateral vessels. (correct answer)
  4. Higher baseline stores of intracellular glycogen and creatine phosphate.
Explanation: The correct answer is C. Chronic, repetitive episodes of ischemia, such as those occurring in stable angina, are a potent stimulus for angiogenesis and the development of collateral circulation. These are small arterial connections that grow between different coronary artery territories. When an acute occlusion occurs, these pre-existing collateral vessels can provide an alternative route for blood flow to the ischemic myocardium, thereby limiting the size of the resulting infarction.

Question 17

Within the first 60 seconds of a total coronary artery occlusion, cardiomyocytes lose their ability to contract. This rapid contractile failure, which occurs long before irreversible cell death, is primarily caused by:

  1. Massive influx of extracellular calcium through damaged sarcolemma, leading to hypercontraction.
  2. Depletion of ATP, which impairs both sarcoplasmic reticulum calcium handling and actin-myosin cross-bridge detachment. (correct answer)
  3. Accumulation of intracellular lactate, which directly denatures contractile proteins and causes immediate rigor.
  4. Efflux of intracellular potassium, leading to hyperpolarization and inactivation of voltage-gated calcium channels.
Explanation: The correct answer is B. Loss of contractility is one of the earliest functional consequences of myocardial ischemia, occurring within a minute. It is directly tied to the rapid depletion of ATP. ATP is required for the sarcoplasmic reticulum Ca2+-ATPase (SERCA) pump to sequester calcium, and it is also required for the detachment of myosin heads from actin filaments. Failure of both processes leads to contractile dysfunction well before the cell membrane ruptures or irreversible injury occurs.

Question 18

Approximately 7 to 10 days after a myocardial infarction, the infarcted tissue is maximally soft and fragile. What two cellular processes are concurrently peaking during this period to create this state?

  1. Phagocytosis by macrophages and the formation of new granulation tissue. (correct answer)
  2. Neutrophil infiltration and coagulative necrosis.
  3. Deposition of dense collagen and apoptosis of fibroblasts.
  4. Interstitial edema and wavy fiber formation.
Explanation: When you encounter questions about myocardial infarction healing, think about the timeline of tissue repair and the specific cellular events occurring at each stage. The 7-10 day period represents a critical transition phase in cardiac healing. At this timepoint, two key processes create maximum tissue vulnerability. First, macrophages are actively phagocytosing (eating) the dead cardiac muscle tissue from the initial infarct. This cleanup process literally removes structural tissue, creating gaps and weakening the myocardial wall. Simultaneously, new granulation tissue is forming - this consists of new capillaries, fibroblasts, and loose connective tissue that will eventually mature into scar tissue. However, this early granulation tissue is inherently fragile and lacks the structural integrity of normal myocardium or mature scar tissue. Answer A correctly identifies both processes that peak during this vulnerable period. Answer B describes events from the first few days post-MI - neutrophils dominate the initial inflammatory response (days 1-3), while coagulative necrosis occurs immediately after vessel occlusion. Answer C describes much later events - dense collagen deposition happens weeks later during scar maturation, not at 7-10 days when collagen is still loose and disorganized. Answer D refers to very early changes - interstitial edema and wavy fiber formation occur within hours to days of infarction. Remember that cardiac rupture risk peaks at 7-10 days precisely because of this combination of tissue removal and immature replacement tissue. Focus on understanding the temporal sequence of healing phases when studying MI pathophysiology.

Question 19

Within the first 60 seconds of a total coronary artery occlusion, cardiomyocytes lose their ability to contract. This rapid contractile failure, which occurs long before irreversible cell death, is primarily caused by:

  1. Massive influx of extracellular calcium through damaged sarcolemma, leading to hypercontraction.
  2. Depletion of ATP, which impairs both sarcoplasmic reticulum calcium handling and actin-myosin cross-bridge detachment. (correct answer)
  3. Accumulation of intracellular lactate, which directly denatures contractile proteins and causes immediate rigor.
  4. Efflux of intracellular potassium, leading to hyperpolarization and inactivation of voltage-gated calcium channels.
Explanation: The correct answer is B. Loss of contractility is one of the earliest functional consequences of myocardial ischemia, occurring within a minute. It is directly tied to the rapid depletion of ATP. ATP is required for the sarcoplasmic reticulum Ca2+-ATPase (SERCA) pump to sequester calcium, and it is also required for the detachment of myosin heads from actin filaments. Failure of both processes leads to contractile dysfunction well before the cell membrane ruptures or irreversible injury occurs.

Question 20

A patient undergoes successful reperfusion therapy for an acute MI. However, post-procedure monitoring shows a paradoxical worsening of tissue damage, a phenomenon known as reperfusion injury. A key mechanism of this injury is the massive influx of intracellular Ca2+ upon re-establishment of blood flow. This calcium overload is exacerbated by:

  1. ATP-depletion-induced failure of the Na+/K+ pump, which leads to reversal of the Na+/Ca2+ exchanger. (correct answer)
  2. Rapid washout of lactate, which causes intracellular alkalosis and activates calcium channels.
  3. The restored blood flow delivering high concentrations of calmodulin, which binds and sequesters calcium.
  4. Increased expression of sarcoplasmic reticulum Ca2+-ATPase (SERCA) pumps, which overload the SR with calcium.
Explanation: The correct answer is A. During ischemia, ATP depletion causes the Na+/K+-ATPase to fail, leading to a buildup of intracellular Na+. To compensate, the Na+/Ca2+ exchanger, which normally exports Ca2+, reverses its direction and begins importing Ca2+ in exchange for exporting Na+. Upon reperfusion, this dysfunctional state persists and is exacerbated, leading to a massive, toxic influx of Ca2+ that activates degradative enzymes and contributes to cell death.