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Lysosomal storage diseases form a category of genetic

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Lysosomal storage diseases form a category of genetic [#permalink] New post 25 Jun 2012, 18:47
Lysosomal storage diseases form a category of genetic disorders resulting from defective enzymes that normally function to break down unneeded molecules in cells. These enzymes do their work in the lysosome, a small compartment in a cell analogous to a garbage disposal. When any one of the lysosomal enzymes is defective, the molecules that would have been broken down by that enzyme instead accumulate and cause that individual's lysosomes to swell enormously, resulting in motor and mental deterioration, often to the point of premature death. The age of onset, rate of progression, and severity of the clinical symptoms observed in patients with the same defective lysosomal enzyme are highly variable. For many years, this variability in patients with the same defective enzyme puzzled scientists. Only recently have researchers begun to answer the riddle, thanks to a genetic analysis of a particular lysosomal storage disorder known as Tay-Sachs disease.

As in most lysosomal storage diseases, patients suffering from Tay-Sachs disease show both mental and motor deterioration and variability in age of onset, progression, and severity. Physicians have categorized the patients into three groups based on onset of the disease: infantile, juvenile, and adult. The infantile group begins to show neurodegeneration as early as six months of age and children rarely live beyond 3 years old. The first symptoms of the disease appear in juvenile cases between 2 and 5 years of age, with death usually occurring around age 15. Those with the adult form generally live out a normal lifespan, suffering from milder symptoms than those with the infantile and juvenile forms.

In Tay-Sachs disease, scientists were aware that molecules accumulated mainly in the brains of patients, but they did not discover the specific identity of the defective lysosomal enzyme responsible for the malfunction, hexosaminidase, until the 1960s. In 1985, the DNA sequence for the normal enzyme was determined. Shortly thereafter, the DNA sequences of genes encoding hexosaminidase from many Tay-Sachs patients were studied. It soon became apparent that not one or two but many different types of mutations in the hexosaminidase gene could result in Tay-Sachs disease. These different mutations resulted in various levels of impaired enzymatic activity; those in the infantile category had little to no normal activity, while those in the adult category suffered only moderate impairment. Scientists quickly hypothesized that the variation in age of onset and severity of Tay-Sachs disease correlated with the amount of residual enzymatic activity allowed by the particular genetic mutation a patient had. Though more research is needed to demonstrate similarity with other lysosomal storage diseases, the work done on Tay-Sachs disease has already offered a promising glimpse into the underlying mechanisms of these disorders.
1. The passage suggests that which of the following lines of inquiry would be most useful in determining the relevance of the research done on Tay-Sachs disease to lysosomal storage diseases generally?
<>Do patients suffering from other lysosomal storage diseases have the same mortality rate as those suffering from Tay-Sachs?
<>Do other lysosomal storage diseases affect the hexosaminidase gene?
<>How many different mutations are present in the defective genes responsible for other lysosomal storage diseases?
<>Does the age of onset or severity of other lysosomal storage diseases vary with the specific genetic mutation observed in the patient?
<>What purpose does hexosaminidase serve in the human body?
[Reveal] Spoiler:
D


2. It can be inferred from the passage that which of the following statements is true of lysosomal storage diseases?
They are generally caused by mutations to the hexosaminidase gene.
They are undetectable until physical symptoms are present.
They can be fatal even when allowing some enzymatic activity
They are most lethal when onset is in a patient's infancy.
Their causes were unknown before the 1950s.
[Reveal] Spoiler:
C


3. The author of the passage is primarily concerned with
<>illuminating the physiological consequences of Tay-Sachs disease
<>explaining the importance of research on a specific disease to other diseases of that type
<>arguing for a more detailed examination of lysosomal storage diseases
<>challenging a traditional view of a class of diseases as incomplete
<>describing the implications of genetic mutations for mortality rates
[Reveal] Spoiler:
B

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Re: manhattan cat 2 v 20 [#permalink] New post 25 Jun 2012, 18:54
OA is DCB.

I have a question regarding Q2.

This is explanation provided by Manhattan, but I don't agree with it, especially red parts.


(A) This choice states that it can be inferred that lysosomal storage diseases "are generally caused by mutations to the hexosaminidase gene." The passage only states that mutations in this gene are responsible for Tay-Sachs disease. There is no information suggesting that it is responsible for the entire class of storage diseases.


(B) This choice states that it can be inferred that lysosomal storage diseases "are undetectable until physical symptoms are present." The passage, however, discusses some specific genetic mutations that are linked to Tay Sachs diseases, a type of lysosomal storage disease. This indicates, if anything, that the disease might be detectable before physical symptoms are present.


(C) CORRECT. This choice states that it can be inferred that lysosomal storage diseases "can be fatal even when allowing some enzymatic activity." This can be inferred from combining two statements in the passage: paragraph 3, which states that “those in the infantile category had little to no normal activity” and paragraph 2, which states that “children [with the infantile form of the disease] rarely live beyond 3 years old.” "Little... activity," while low, is still not zero activity.

(D) This choice states that it can be inferred that lysosomal storage diseases "are most lethal when onset is in a patient's infancy." The information in the passage relating to infant mortality is given in the specific context of Tay-Sachs disease. We cannot know whether this pattern holds true for other diseases of this type. In addition, while those with the infantile form do die of the disease, so do those with the juvenile form, making the disease equally lethal.

(E) Choice E states that it can be inferred that the causes of lysosomal storage diseases were unknown before the 1950s. The passage states, however, that the causes of Tay-Sachs disease were unknown before that time. We do not have any information about the causes of lysosomal storage diseases generally.

The explanation says that the reason that A and D are not the answer is that A and D can be applied only to Tay-Sachs disease.
But dosen't the explanation for C also talk about Tay-Sachs disease ? “those in the infantile category had little to no normal activity”
children [with the infantile form of the disease] rarely live beyond 3 years old.”
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Re: manhattan cat 2 v 20 [#permalink] New post 26 Jun 2012, 01:03
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My Answers were DBB for above package, but for 2nd question after reading explanation,

I think C is the correct answer as per explanation provided "little to no normal activity".. I find this logical statement means there is some activity is allowed children do not live beyond 3 years. if we ignore this inference then statement made in above paragraph fails.

I agree with Manhattan explanation
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Re: Lysosomal storage diseases form a category of genetic [#permalink] New post 30 Jun 2012, 00:01
i guess, this would clear your doubt.

When any one of the lysosomal enzymes is defective, the molecules that would have been broken down by that enzyme instead accumulate and cause that individual's lysosomes to swell enormously, resulting in motor and mental deterioration, often to the point of premature death
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Re: Lysosomal storage diseases form a category of genetic   [#permalink] 30 Jun 2012, 00:01
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