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## Manhattan RC: Toxoplasma gondii (The trickiest RC so far)

tagged by: ceilidh.erickson

This topic has 3 expert replies and 0 member replies
joealam1 Senior | Next Rank: 100 Posts
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#### Manhattan RC: Toxoplasma gondii (The trickiest RC so far)

Tue Nov 29, 2016 10:55 pm
The single-celled parasite known as Toxoplasma gondii infects more than half of the world's human population without creating any noticeable symptoms. Once inside the human body, Toxoplasma rapidly spreads to the heart and other organs. It can even penetrate the tight barrier that normally protects the brain from most pathogens. Yet, the blood of infected persons carries very few free-floating Toxoplasma cells. Scientists have long been puzzled by this ability of Toxoplasma to parasitize the human body without triggering an immune response and without an appreciable presence in the bloodstream. Recent research, however, has shed light on the ways in which Toxoplasma achieves its remarkable infiltration of the human body.
Though there are few individual Toxoplasma cells coursing freely in the blood of an infected person, scientists have discovered that the parasite is quite common in certain cells, known as dendritic cells, involved in the human immune system. Dendritic cells are found in the digestive tract and frequently come into contact with the various pathogens that enter the human body through food and water. When the dendritic cells encounter pathogens, they travel to lymph nodes and relay this information to other immune cells that then take action against the reported pathogen. Scientists have found, however, that Toxoplasma is capable of hijacking dendritic cells, forcing them from their usual activity and using them as a form of transportation to infect the human body quickly. Without this transport mechanism, Toxoplasma could not reach the better-protected areas of the body.
Toxoplasma invades the human body through consumption of the undercooked meat of infected animals, primarily pigs and chickens. Other animals, such as cats, can become infected as well. In fact, cats are a necessary component in the reproductive cycle of Toxoplasma, since the animal's intestines are the parasite's sole breeding ground. Toxoplasma creates egg-like cysts, known as oocysts, in the cats' intestines. These oocysts are shed in the cats' droppings and contaminate ground water and soil, eventually finding their way into the food chain. Because Toxoplasma must somehow find its way into a new host cat in order to reproduce, it cannot kill its current host. Instead, it waits for the host, usually a small rodent, to be eaten by a cat, thus providing Toxoplasma the opportunity to reproduce.

1) It can be inferred from the passage that which of the following statements is true of dendritic cells in the human body?
A) They are produced by the lymphatic system.
B) They are more numerous in the digestive tract than in any other part of the human body.
C) Most dendritic cells of persons infected with Toxoplasma carry the parasite.
D) They are the only cells capable of being infected by Toxoplasma.
E) They are able to penetrate the membranes surrounding the brain.

OA:E
I didn't get why B is wrong, it is mentioned clearly in the passage that"Dendritic cells are found in the digestive tract" which means that they are not found elsewhere

2)According to the passage, all of the following are true of Toxoplasma gondii EXCEPT
A) it can contaminate ground water
B) it enters the human body through the food chain
C) it can alter the usual behavior of human cells
D) the human body is incapable of detecting it
E) it must find a host cat in order to reproduce

OA: D

3)The second paragraph performs which of the following functions in the passage?
A) It describes a paradox that scientists were unable to resolve for some time.
B) It explains the effectiveness of a lethal biological pathogen.
C) It describes the mechanism by which a biological event occurs in humans.
D) It demonstrates how the infectious agent reproduces itself.
E) It introduces information that is essential to understanding the role of Toxoplasma in human development.

OA: C

on the 1st paragprah the author mentioned at the end that new research has helped in unraveling the paradox, paragraph 2 is explaining this paradox so why A is wrong?

### GMAT/MBA Expert

ceilidh.erickson GMAT Instructor
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Sat Dec 10, 2016 12:14 pm
Hey joealam1,

(In the future, please post only 1 question per thread, to make it easier for other students to search).

Quote:
1) It can be inferred from the passage that which of the following statements is true of dendritic cells in the human body?

A) They are produced by the lymphatic system.
B) They are more numerous in the digestive tract than in any other part of the human body.
C) Most dendritic cells of persons infected with Toxoplasma carry the parasite.
D) They are the only cells capable of being infected by Toxoplasma.
E) They are able to penetrate the membranes surrounding the brain.
Here's what the passage tells us about dendritic cells, and what we can infer:

- "parasite is quite common in certain cells, known as dendritic cells, involved in the human immune system" --> we can't infer much beyond this. The parasite might also be common in other cells, and dendritic cells might be involved in other things.

- "Dendritic cells are found in the digestive tract and frequently come into contact with the various pathogens that enter the human body through food and water." --> this does NOT mean that they're not found elsewhere. All we can infer is that this is where they encounter the parasite.

- "When the dendritic cells encounter pathogens, they travel to lymph nodes and relay this information to other immune cells" --> we can infer the dendritic cells travel OUTSIDE of the digestive tract, and that they communicate with other cells.

- "Toxoplasma is capable of hijacking dendritic cells, forcing them from their usual activity and using them as a form of transportation to infect the human body quickly." --> dendritic cells have a "usual" activity, they can be hijacked, they can transport toxoplasma throughout the body.

- "Without this transport mechanism, Toxoplasma could not reach the better-protected areas of the body." --> This tells us that dendritic cells are solely responsible for toxoplasma reaching "better-protected' areas. The only such protection mentioned was "the tight barrier that normally protects the brain from most pathogens," so we can assume that this is what the author is referring to.

When answering an INFERENCE question, we ask ourselves, "does this HAVE to be true?" Look out especially for DEALBREAKER words: words that are too definitive to be true in every case.

A) They are produced by the lymphatic system.
They TRAVEL to the lymph nodes, but we are given no indication about where they are produced.

B) They are more numerous in the digestive tract than in any other part of the human body.
We're given no information to compare where they are most numerous. To your question:
Quote:
it is mentioned clearly in the passage that"Dendritic cells are found in the digestive tract" which means that they are not found elsewhere
this is not an accurate inference. If I said "children are found in schools," you wouldn't conclude that they couldn't be found elsewhere. In fact, we're told that the parasite hijacks dendritic cells and travels to other parts of the body, so dendritic cells MUST be found in other parts of the body.

C) Most dendritic cells of persons infected with Toxoplasma carry the parasite.
This does not have to be true. Perhaps only a small handful of infected dendritic cells are enough to infect a person. (The word "most" is usually a dealbreaker).

D) They are the only cells capable of being infected by Toxoplasma.
This may be true, but it doesn't have to be true. Just because we're told that X does something, that doesn't mean that Y doesn't do that thing. The word "only" is usually a dealbreaker.

E) They are able to penetrate the membranes surrounding the brain.
If dendritic cells are solely responsible for getting toxoplasma to the "better-protected areas," which we can assume means the brain, then dendritic cells must be able to gain entry into the brain. This must be true.

The answer is E.

_________________

Ceilidh Erickson
Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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### GMAT/MBA Expert

ceilidh.erickson GMAT Instructor
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Sat Dec 10, 2016 12:21 pm
Quote:
2)According to the passage, all of the following are true of Toxoplasma gondii EXCEPT:

A) it can contaminate ground water
B) it enters the human body through the food chain
C) it can alter the usual behavior of human cells
D) the human body is incapable of detecting it
E) it must find a host cat in order to reproduce
For EXCEPT questions, the only strategy is to go through each answer choice, and check it against the passage. Do not simply rely on your memory, since there may be things that sound familiar, but were not explicitly stated in the passage.

A) it can contaminate ground water
"oocysts are shed in the cats' droppings and contaminate ground water and soil,"
True.

B) it enters the human body through the food chain
"eventually finding their way into the food chain"
"Toxoplasma invades the human body through consumption of the undercooked meat of infected animals"

True.

C) it can alter the usual behavior of human cells
"Toxoplasma is capable of hijacking dendritic cells, forcing them from their usual activity"
True.

D) the human body is incapable of detecting it
We know that it infects the body "without triggering an immune response," but this does not mean that it was undetected. It was simply not responded to.
Not necessarily true.

E) it must find a host cat in order to reproduce
"cats are a necessary component in the reproductive cycle of Toxoplasma, since the animal's intestines are the parasite's sole breeding ground"
True.

The answer is D.

_________________

Ceilidh Erickson
Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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### GMAT/MBA Expert

ceilidh.erickson GMAT Instructor
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Sat Dec 10, 2016 12:31 pm
Quote:
3)The second paragraph performs which of the following functions in the passage?

A) It describes a paradox that scientists were unable to resolve for some time.
B) It explains the effectiveness of a lethal biological pathogen.
C) It describes the mechanism by which a biological event occurs in humans.
D) It demonstrates how the infectious agent reproduces itself.
E) It introduces information that is essential to understanding the role of Toxoplasma in human development.
When we're asked the FUNCTION of a particular paragraph or sentence, we should think of it in relation to what comes before or after. Here are some of the possible functions:
- introduce a new idea
- provide an example of an idea previously presented
- provide an explanation
- draw a conclusion
etc.

If I were to sketch a rough outline for the function of each paragraph, it might look like this:
I. introduce a puzzling phenomenon about the behavior of a parasite
II. detail how the parasite behaves in the human body, explaining the puzzling phenomenon
III. explain the larger life cycle of the parasite outside of the human body

A) It describes a paradox that scientists were unable to resolve for some time.
The paradox is infection without an immune system reaction. This was described in the 1st paragraph, not the 2nd. The 2nd gives the explanation, not the paradox itself.

B) It explains the effectiveness of a lethal biological pathogen.
The word "effectiveness" is a problem here. This paragraph explains how the parasite travels without triggering the immune system, but not what makes it lethal.

C) It describes the mechanism by which a biological event occurs in humans.
Exactly. It details the "how" in the human body.

D) It demonstrates how the infectious agent reproduces itself.
No, this happens in the 3rd paragraph.

E) It introduces information that is essential to understanding the role of Toxoplasma in human development.
This has nothing to do with human development, only the behavior of certain human cells.

The answer is C.

_________________

Ceilidh Erickson
Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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