Homocystinuria and its associate treatments
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- Homocystinuria and its associate treatments
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Homocystinuria and its associate treatments
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[ID] => 560250
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[post_date] => 2025-01-14 06:00:03
[post_date_gmt] => 2025-01-14 11:00:03
[post_content] => Practice Passage (Question 1-5)
*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.
Homocystinuria is an inherited disorder of the metabolism of the amino acid methionine. The normal metabolism of this substance requires the enzyme cystathionine beta synthase. The disease is inherited as an autosomal recessive trait, with approximately 1 in 100,000 infants being born with homocystinuria.
In individuals with homocystinuria, homocysteine, an amino acid, accumulates in the serum, leading to an increased excretion of homocysteine in the urine. High levels of homocysteine in the blood results in a multi-systemic disorder of the connective tissue, muscles, central nervous system, and the cardiovascular system. Fortunately, homocystinuria can be identified by a blood test at birth, which allows for preemptive treatment to be administered to prevent dramatic damage to the individual’s body. One way symptoms of the disease can be avoided is through a strictly reduced methionine diet. Another treatment comes in the form of high doses of pyridoxine, or vitamin B6.
[post_title] => Homocystinuria and its associate treatments
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[question] => Since the laboratory analysis of homocysteine is complicated by the fact that it exists bound to other thiol amino acids via disulfide bonds, it is first necessary to subject these disulfide bonds to reduction prior to analysis. It is necessary to reduce these disulfide bonds in order to get a true value of the quantity of homocysteine in a plasma sample. Why is this so?
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[description] => Reason for the Correct Answer:
Homocysteines that are bound via disulfide bonds do not register as isolated homocysteine molecules.
Reduction of disulfide bonds separates homocysteine molecules that are bound to other amino acids.
If the reduction step were skipped, then the true quantity of homocysteine would be artificially low.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.The true quantity of homocysteine would be artificially high if the reduction step were not carried out prior to analysis.
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[1] => Array
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[each_answer] => B.It is necessary because disulfide bound homocysteine would otherwise precipitate out of the serum before analysis could be conducted.
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[2] => Array
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[each_answer] => C.The true quantity of homocysteine would be artificially low if the reduction step were not carried out prior to analysis.
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[each_answer] => D.It is necessary in order to prevent homocysteine from degrading in the sample.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => If each member of a couple is heterozygous for homocystinuria, and they have a child, what is the probability that the child will have at least one normal allele?
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[answer] => 2
[description] => Reason for the Correct Answer:
If both parents are heterozygous, then they have each have a 50% chance of passing on the abnormal allele.
A child that is either homozygous for the normal allele or heterozygous will have at least one normal allele.
There is a 25% chance (Remember, 50% x 50% = 25%) that a child will receive both abnormal alleles from the parents, leaving a 75% chance that the child will have at least one normal allele. Alternatively, you could do a punnett square analysis to come with the same answer.
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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.75%
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[each_answer] => C.50%
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[each_answer] => D.0%
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Most serum homocysteine exists in a form bound to other thiol amino acids and proteins in the form of disulfides (e.g. cystine-homocystine and homocystine-homocystine). Disulfide bonds play a role in which levels of protein structure?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
Primary structure is determined by covalent peptide bonds.
Secondary structure is determined by hydrogen bonding between distance side chains of amino acids in the same polypeptide.
Disulfide bonds play a role in determining tertiary and quaternary structure.
)
[answers] => Array
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[0] => Array
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[each_answer] => A.Tertiary structure only
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[1] => Array
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[each_answer] => B.Tertiary and quaternary structure
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[each_answer] => C.Quaternary structure only
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[each_answer] => D.Secondary structure only
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[3] => Array
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[quiz_unique_key] => 2377279144
[question] => A group of investigators would like to create a test that is sensitive enough to pick up a pathological level of homocysteine in an infant’s urine. If the pathological level of homocysteine is considered to be one standard deviation higher than the average level of homocysteine in infant urine, then which threshold level should the investigators choose for their test? (X axis = homocysteine concentration; Y axis = number of urine samples x 1,000)
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[answer] => 3
[description] => Reason for the Correct Answer:
The mean of the samples is between points B and C.
Point A is two standard deviations below the mean, Point B is one standard deviation below the mean, Point C is one standard deviation above the mean, and Point D is two standard deviations above the mean.
Point C is one standard deviation above the mean, and thus that should be the threshold level for the test.
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[each_answer] => A.Point A
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[each_answer] => B.Point B
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[each_answer] => C.Point C
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[each_answer] => D.Point D
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[quiz_unique_key] => 2261298308
[question] => Which of the following describes the solubility of methionine?
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[answer] => 3
[description] => Reason for the Correct Answer:
Methionine is a non-polar, hydrophobic amino acid.
Water is a polar solvent, and lipids are nonpolar.
Thus, methionine will have low solubility in water and high solubility in lipids.
)
[answers] => Array
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[each_answer] => A.Low in both water and lipids.
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[each_answer] => B.High in both water and lipids.
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[2] => Array
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[each_answer] => C.Low in water and high in lipids.
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[each_answer] => D.High in water and low in lipids.
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[560250|1] => C
[560250|2] => B
[560250|3] => B
[560250|4] => C
[560250|5] => C
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