RC: CAT Question: Lysosomal Storage Diseases

[sean.m.hetherington]

Passage Text:
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.

Question and Answer Choices:
It can be inferred from the passage that which of the following statements is true of lysosomal storage diseases?

A) They are generally caused by mutations to the hexosaminidase gene.

B) They are undetectable until physical symptoms are present.

C) They can be fatal even when allowing some enzymatic activity.

D) They are most lethal when onset is in a patient's infancy.

E) Their causes were unknown before the 1950s.

THE CORRECT ANSWER IS C

Explanation:
The question asks broadly which of the choices is a valid inference from the information in the passage. The best approach here is simply to evaluate the choices one by one.

(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.

My Question:
I chose answer choice E, however, the explanation states that answer choice E is wrong because it is referring specifically to Tay-Sachs disease and cannot be generalized to all/other lysosomal storage disease. I understand why E is wrong, though using the same logic that we cannot use evidence specific to Tay-Sachs to generalize for other lysosomal storage disease, C would be wrong as well (at least based on the explanation given).

Can you please help me to understand why answer choice C would be correct and not answer choice E?

Thank you in advance,
Sean

[jnelson0612]

Hi Sean,
Check out this thread, in which another student also asks why the answer is C and not E. mgmat-rc-t4028.html