Passage


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.

Toxoplasma - Except

According to the passage, all of the following are true of Toxoplasma gondii EXCEPT

A it can contaminate ground water[/list]

B it enters the human body through the food chain[/list]

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

I am having trouble understanding the generalization in choice III
The passage says it can alter the usual behaviour of human dendritic cells, whereas Choice III is: it can alter the usual behavior of human cells. IMO, toxoplasma could affect dendritic cells only and may be few other cells, but choice III seems broad as it encompasses all human cells. I dont know how to eliminate it on this basis.

P.S the answer is IV.

Your help is much appreciated.!

-Uma

hi -

i understand what you're saying here, but you're misinterpreting the language of the answer choice.

in general, if you say "X can VERB OBJECTs", or "X has VERBed OBJECTs", or other such statements, then the default interpretation is that this is a true statement if this can happen for any OBJECTs.

in other words, the statement is the same as it would be with the insertion of "some".

here are some examples:
i've eaten pakistani food.
--> this is a true statement if i've ever eaten pakistani food. i don't have to have tried everything on the menu.

i can draw human hands.
--> still a true statement even if i can't draw, say, babies' or elderly people's hands.

etc.

--

you do, however, have a good point. we should probably add the word "some" to the answer choice, just to make it clearer.

i will submit this problem for revision.

Hi Ron,

(edited) My doubt is with the above question. Choice 3 seems to be the right answer.

It is not mentioned anywhere in the passage that Toxoplasma gondii alters the behavior of human/dentritic cells. However, it is mentioned that Toxoplasma uses dendrititic cells for transportation. But it doesn't seem to imply that the cell's behavior is changed as a result.

Please guide me as to what I am missing here. Thanks!

but it does say that the bacterium changes the usual behavior of the cells. see here:
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.

the boldfaced words prove that the bacterium is actually changing the cell's normal behavior.