Detection of myocardial infarction
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- Detection of myocardial infarction
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Detection of myocardial infarction
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[ID] => 554840
[post_author] => 12815
[post_date] => 2024-12-23 18:31:03
[post_date_gmt] => 2024-12-23 23:31:03
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
The 2007 American College of Cardiology (ACC) definition of myocardial infarction includes the detection of a rise and/or fall of cardiac biomarkers – with at least one value exceeding the 99th percentile of the upper reference limit – with evidence of myocardial damage. Cardiac enzymes that are elevated following myocardial injury include troponin and creatinine kinase.
In the cell, there are three troponin subunits: Troponin Inhibitory unit (TnI), Troponin Tropomyosin binding unit (TnT) and Troponin Calcium binding unit (TnC). Troponin is found in both cardiac and skeletal muscles. Cardiac TnI (cTnI) contains 31 additional amino acid residues on its N-terminus compared with TnI in skeletal muscles. TnT also exists in different forms in cardiac and skeletal muscles, whereas the TnC forms found in skeletal and cardiac muscles are identical. Diagnostic assays use antibodies that are specific for cardiac TnI (cTnI) and cardiac TnT (cTnT); however, cTnT has been shown to also be upregulated in conditions such as muscular dystrophy and renal failure. The detection limit of both cTnI and cTnT levels for most assays is 0.01 ng/mL, and the 99th percentile of these biomarkers in the normal population is reported as 0.04 ng/mL.
Creatine kinase consists of two chains (M and B) that form three isozymes: CK-MB, CK-MM and CK-BB. CK-MM and CK-MB are found in skeletal muscles, so skeletal muscle disease will be associated with elevated CK-MM and CK-MB levels.
Other biomarkers of myocardial Infarction that are no longer used or have uncertain clinical relevance include lactate dehydrogenase, myoglobin, carbonic anhydrase, glycogen phosphorylase, heart fatty acid binding protein, and ischemia-modified albumin.
Figure 1 shows how the levels of cardiac enzymes change in the 160 hours following myocardial infarction.
Figure 1 Factor increase of cardiac enzyme levels above normal during the 160 hours following myocardial infarction.
Passage information and figure adapted from UPMC, Final Diagnosis -- Myocardial Infarction, https://path.upmc.edu/cases/case735/dx.html
[post_title] => Detection of myocardial infarction
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[quiz_unique_key] => 602779517
[question] => According to the information in the passage, what likely makes TnI a more useful biomarker than TnC for the detection of myocardial infarction?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The passage states that cTnI contains an additional 31 amino acid residues on the N-terminus compared to TnI in skeletal muscle, whereas TnC forms found in skeletal and cardiac muscles are identical.
This is the advantage for analyzing TnI, because an assay that detects cTnI would be specific to cTnI and wouldn’t also pick up TnI released from skeletal muscle cells. Because cTnI and TnI are expressed in different tissues, this would make the test more specific for myocardial damage.
cTnI having 31 fewer amino acid residues could be explained by cTnI being coded by a different gene than skeletal TnI. (This could also be explained by alternative splicing, a post-transcriptional modification that occurs before translation.)
)
[answers] => Array
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[0] => Array
(
[each_answer] => A. TnC undergoes different post-translational modifications compared to TnC expressed in skeletal muscle.
)
[1] => Array
(
[each_answer] => B. TnI is encoded by different genes in cardiac and skeletal muscle.
)
[2] => Array
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[each_answer] => C. TnI levels also rise following damage to skeletal muscle.
)
[3] => Array
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[each_answer] => D. Only TnI is released into the bloodstream following myocardial damage.
)
)
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => The passage suggests that troponin levels peak around what level in the hours following a myocardial infarction?
[value] => Array
(
[answer] => 3
[description] => Reason for Correct Answer:
The graph shows troponin levels increasing to about 5.5x the upper limit of normal.
The passage states that “the 99th percentile of [cTnI and cTnT] biomarkers in the normal population is reported as 0.04 ng/mL,” and this should be the “upper limit of normal.”
Therefore, troponin levels rise to about 5.5 x 0.04 ng/mL = 0.22 ng/mL.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. 0.005 ng/mL
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[1] => Array
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[each_answer] => B. 0.006 ng/mL
)
[2] => Array
(
[each_answer] => C. 0.22 ng/mL
)
[3] => Array
(
[each_answer] => D. 0.26 ng/mL
)
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => Which description fits the enzyme that increases the earliest in the peripheral bloodstream following ischemic injury?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
Myoglobin and hemoglobin are both important for oxygen transport.
Hemoglobin is a tetrameric protein that exhibits cooperative binding and release of oxygen molecules; it carries oxygen in red blood cells throughout the circulation.
Myoglobin, however, is found in muscle cells.
It is a monomeric protein that binds to oxygen molecules and stores them within muscle cells. As the oxygen levels decrease, such as during strenuous exercise, myoglobin releases the stored oxygen to be used by the mitochondria within the muscle cells for energy production through aerobic respiration. Myoglobin has a higher affinity for oxygen compared with hemoglobin, which enables it to effectively extract oxygen from the bloodstream and maintain a supply within muscle tissues, even when blood oxygen levels are low. This property is essential for sustaining muscle function and performance during various physiological conditions.
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[answers] => Array
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[0] => Array
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[each_answer] => A. A monomeric protein that facilitates oxygen storage and release in muscles
)
[1] => Array
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[each_answer] => B. A tetrameric protein that aids in oxygen delivery to tissues throughout the body
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[2] => Array
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[each_answer] => C. A dimeric protein that exhibits cooperative binding of oxygen molecules
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[3] => Array
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[each_answer] => D. A transport protein found exclusively in cardiac muscle cells
)
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[3] => Array
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[quiz_unique_key] => 1403770772
[question] => What is a likely explanation for why lactate dehydrogenase (LDH) is seldom used in the clinical setting to detect the onset of myocardial infarction?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
The graph shows that lactate dehydrogenase can be detected in the bloodstream following myocardial infarction.
It also shows that it starts to rise right after an MI, but doesn’t peak until later than the other biomarkers.
However, LDH is involved in many cellular processes, and LDH levels in the blood could indicate damage to other tissues and organs, not necessarily the heart. Other protein levels would need to be measured as well.
)
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[each_answer] => A. It is not specific for cardiac tissue injury.
)
[1] => Array
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[each_answer] => B. It peaks too early after the initial ischemic insult.
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[2] => Array
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[each_answer] => C. It does not accumulate to a concentration that can be detected in the blood.
)
[3] => Array
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[each_answer] => D. Its appearance in the bloodstream is unrelated to cardiac tissue injury.
)
)
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[4] => Array
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[quiz_unique_key] => 1403770772
[question] => In muscle cells, what does the binding of calcium to TnC accomplish?
[value] => Array
(
[answer] => 4
[description] => Reason for Correct Answer:
Troponin is normally associated with another protein, tropomyosin, and together, they help control muscle contraction.
In the absence of calcium, tropomyosin blocks the interaction between actin and myosin filaments, preventing muscle contraction.
When calcium binds to troponin, it induces a conformational change in the troponin complex, which also affects the positioning of tropomyosin.
This change allows tropomyosin to move away from its inhibitory position on the actin filament, no longer blocking the myosin -binding sites. As a result, myosin is allowed to bind to actin, forming cross-bridges, and initiating muscle contraction
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/troponin-c
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[answers] => Array
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[0] => Array
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[each_answer] => A. Consumption of ATP to break actin–myosin cross-bridges
)
[1] => Array
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[each_answer] => B. Removal of the TnT from its actin binding site
)
[2] => Array
(
[each_answer] => C. Movement of TnI to allow tropomyosin to bind to myosin
)
[3] => Array
(
[each_answer] => D. Reversal of the tropomyosin inhibition of myosin binding to actin
)
)
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[5] => Array
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[quiz_unique_key] => 1997864699
[question] => How much longer would the protein-coding region of the cTnI gene be relative to the protein-coding region of the gene for skeletal muscle TnI?
[value] => Array
(
[answer] => 1
[description] => Reason for Correct Answer:
The passage states that cTnI contains an additional 31 amino acid residues at the N-terminus compared with TnI in skeletal muscle.
Each amino acid is encoded by 3 base pairs.
Therefore, the protein-coding region of cTnI would be 31 x 3 = 93 base pairs longer.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. 93 base pairs
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[1] => Array
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[each_answer] => B. 103 base pairs
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[2] => Array
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[each_answer] => C. 31 base pairs
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[3] => Array
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[each_answer] => D. 62 base pairs
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[554840|1] => B
[554840|2] => C
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[554840|4] => A
[554840|5] => D
[554840|6] => A
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Figure 1
