Duchenne muscular dystrophy
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Duchenne muscular dystrophy
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[ID] => 554609
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[post_date] => 2024-12-23 18:15:05
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[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.
Dystrophin is an important structural protein in skeletal (striated) muscle. It is located in the cytoplasm, where it helps connect actin filaments in the cytoskeleton of muscle fibers to the extracellular matrix (ECM), as shown in Figure 1. Dystrophin is important for maintaining the mechanical stability of skeletal muscle.
Figure 1 The role of dystrophin in muscle structure and integrity
Duchenne muscular dystrophy (DMD) is a genetic condition caused by a mutation in the gene coding for dystrophin. In DMD, the absence of dystrophin causes muscle cells to be increasingly fragile and prone to membrane damage. The result of this damage is a steady leak of calcium into the sarcolemma, disrupting signaling pathways and ultimately causing mitochondria to burst. Damaged muscle tissue is progressively replaced by scar tissue and fat.
DMD typically presents around the age of three, first presenting as pain and weakness affecting the muscles of the hips, thighs, and shoulders, and then progressing to the arms, legs, and trunk. Other symptoms include awkward gait, frequent falls, fatigue, poor progression of motor skills, and enlargement of the tongue and calf muscles. The average life expectancy of DMD patients is 25 years.
A 26-year-old pregnant woman is concerned about the risk of DMD in her unborn child. She has an eight-year-old nephew who was recently diagnosed with the condition and an uncle who died before she was born and had a “developmental disorder” with symptoms similar to those described in DMD patients. To help assess the risk to her child, her doctor helps the woman develop a family pedigree, as shown in Figure 2.
Figure 2 Pedigree showing DMD cases and carriers in patient’s family
[post_title] => Duchenne muscular dystrophy
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[question] => Based on the information in the passage and Figure 1, what mode of inheritance characterizes DMD?
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[answer] => 1
[description] => Reason for Correct Answer:
In the pedigree, only males appear to be affected.
Affected males are the children of mothers who are obligate carriers, and roughly 50% of the children of an obligate carrier have the mutated gene.
Females can carry the gene without being affected.
DMD is characterized by an X-linked recessive inheritance pattern. Males have only one X chromosome, so they only need one copy of the disease allele to have the disease.
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[answers] => Array
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[0] => Array
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[each_answer] => A. X-linked recessive
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[each_answer] => B. Autosomal dominant
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[each_answer] => C. Autosomal recessive
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[each_answer] => D. X-linked dominant
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => If the pedigree in Figure 2 is an accurate description of this patient’s family, what is the probability that the indicated female patient’s child will have DMD?
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
We do not know if the mother (our patient) received the gene.
The patient’s mother is an obligate carrier, thus, there is a 50% chance that our patient received the gene from her.
If the patient is a carrier of the gene, there is a 50% chance that she would pass it on to a child.
There is also a 50% chance that the child is male. (If they are female, they will not be affected by the diseased allele because they will likely have a normal copy from their father.)
Therefore, the chance of disease is 50% (mother is carrier) x 50% (she passes it to child) x 50% (child is boy) = 12.5%.
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[answers] => Array
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[0] => Array
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[each_answer] => A. 100%
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[each_answer] => B. 50%
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[each_answer] => C. 25%
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[each_answer] => D. 2.5%
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[quiz_unique_key] => 1403770772
[question] => If light microscopy of a muscle biopsy from a patient affected with DMD revealed that some recognizable dystrophin was present in their cells but that it appeared misshapen, what type of mutation is most likely to have caused their disease?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
A silent mutation is a substitution of a nucleotide that does not result in a change in the amino acid sequence of the encoded protein. This mutation would result in the synthesis of a fully functional protein.
A frameshift mutation is a deletion or insertion of a number of nucleotides that is not divisible by three; a single base deletion usually results in a frameshift mutation. Mutations like this often result in radically altered amino acid sequences.
A missense mutation is a single nucleotide substitution that results in a codon that codes for a different amino acid. Mutations such as this often still allow for translation of the full-length protein; however, depending on the new amino acid, the normal protein structure or function may be modified.
The question states that some recognizable dystrophin is visualized, although it is misshapen. This would most likely result from a missense mutation, which does not change the entire protein but can influence folding interactions.
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[answers] => Array
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[each_answer] => A. Silent mutation
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[each_answer] => B. Missense mutation
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[each_answer] => C. Frameshift mutation
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[each_answer] => D. Single base deletion
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[question] => Which of these changes is responsible for many of the symptoms observed in DMD?
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[answer] => 3
[description] => Reason for Correct Answer:
The first paragraph states that dystrophin mutations lead to a steady leak of calcium into the sarcolemma, which is the membrane surrounding a muscle cell/fiber. (Note that calcium is usually higher outside of the cell.)
The paragraph also states that this leak of calcium ultimately causes mitochondria to burst.
Bursting the mitochondria in the muscle cells leads to a deficiency in ATP, which is required for muscle cell contraction. With ongoing loss of contraction, muscle cells atrophy, as seen in DMD.
Reason for Incorrect Answer:
A. Increased intracellular calcium would actually directly cause INCREASED muscle fiber contractions. (In DMD, this leads to overstressed and damaged muscle cells.) It is the loss of ATP that would eventually lead to decreased muscle contractions and atrophy.
B. Increased intracellular calcium levels lead to the binding of calcium ions to troponin, resulting in a change in the troponin-tropomyosin complex and the activation of muscle contraction. Decreased intracellular calcium levels, on the other hand, would not cause decreased binding of troponin to tropomyosin, but rather a return to the blocked state where the active sites on actin are inaccessible to myosin, resulting in muscle relaxation.
D. Defects in fat metabolism are not mentioned in the passage
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[0] => Array
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[each_answer] => A. Increased intracellular calcium, leading to decreased muscle fiber contractions
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[each_answer] => B. Decreased intracellular calcium, leading to decreased binding of troponin to tropomyosin
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[2] => Array
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[each_answer] => C. Decreased ATP availability, leading to progressive atrophy of muscle fibers
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[each_answer] => D. Defects in fat metabolism, leading to fat infiltration into muscle cells
)
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[4] => Array
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[quiz_unique_key] => 1403770772
[question] => In a normal muscle fiber in its resting state, in which two places are calcium levels highest?
I. In the extracellular fluid outside the fiber
II. Inside the cytoplasm of the fiber
III. Within specialized organelles inside the fiber
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The passage describes how leaks in the sarcolemma cause calcium influx into the muscle cell.
This reflects how calcium levels are usually higher outside of the muscle cell (I).
Calcium levels are also high in the sarcoplasmic reticulum (III), the specialized organelle that releases calcium during muscle contraction.
)
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[each_answer] => A. I and II only
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[each_answer] => B. I and III only
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[each_answer] => C. II and III only
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[each_answer] => D. They are equal in all locations.
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[question] => Which of the following may be detected in the bloodstream of individuals affected by DMD, compared to healthy individuals?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Myoglobin is a protein found in muscle cells that is responsible for storing and transporting oxygen within the muscle tissue.
Creatine kinase (CK) is also an enzyme found in muscle cells and other tissues, and it plays a crucial role in the energy metabolism of muscles. Its primary function in muscle tissue is to facilitate the transfer of high-energy phosphate groups between creatine and adenosine triphosphate (ATP).
The passage states that DMD results in damage to muscle fibers, including damage to the sarcolemma (membrane).
The presence of increased CK in the blood would indicate damage to muscle tissue and disruption of the cell membrane. Myoglobin could also be increased in the blood in these patients.
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[answers] => Array
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[each_answer] => A. Elevated calcium levels
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[each_answer] => B. Elevated hemoglobin levels
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[each_answer] => C. Elevated creatine kinase levels
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[each_answer] => D. Depressed myoglobin levels
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Figure 1
Figure 2
