PT141.S3.P1.Q2 > Explanation

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An organism is considered to have an infection when a disease-causing agent, called a pathogen, establishes a viable presence in the organism. This can occur only if the pathogenic agent is able to reproduce itself in the host organism. …

Context · Pathogens must be capable of reproduction

The only agents believed until recently to be responsible for infections—viruses, bacteria, fungi, and parasites—reproduce and regulate their other life processes by means of genetic material, composed of nucleic acid (DNA or RNA). It was thus widely assumed that all pathogens contain such genetic material in their cellular structure.

Earlier view · Pathogens contain DNA or RNA

Because all known pathogens reproduced by DNA or RNA

This assumption has been challenged, however, by scientists seeking to identify the pathogen that causes Creutzfeldt-Jakob disease (CJD), a degenerative form of dementia in humans. CJD causes the brain to become riddled with tiny holes, like a sponge (evidence of extensive nerve cell death). Its symptoms include impaired muscle control, loss of mental acuity, memory loss, and chronic insomnia. Extensive experiments aimed at identifying the pathogen responsible for CJD have led surprisingly to the isolation of a disease agent lacking nucleic acid and consisting mainly, if not exclusively, of protein. Researchers coined the term "prion" for this new type of protein pathogen.

Phenomenon · New pathogen identified with no DNA or RNA

Called a "prion," discovered while studying CJD

Upon further study, scientists discovered that prions normally exist as harmless cellular proteins in many of the body's tissues, including white blood cells and nerve cells in the brain; however, they possess the capability of converting their structures into a dangerous abnormal shape. Prions exhibiting this abnormal conformation were found to have infectious properties and the ability to reproduce themselves in an unexpected way, by initiating a chain reaction that induces normally shaped prions to transform themselves on contact, one after another, into the abnormal, pathogenic conformation. This cascade of transformations produces a plaque, consisting of thread-like structures, that collects in the brain and ultimately destroys nerve cells. …

Hypothesis · Prion reproduction

Prions reproduce by converting converting harmless proteins into dangerous, pathogenic proteins by altering their shape

Because prions, unlike other pathogens, occur naturally in the body as proteins, the body does not produce an immune response when they are present. And in the absence of any effective therapy for preventing the cascade process by which affected prions reproduce themselves, CJD is inevitably fatal, though there are wide variations in pre-symptomatic incubation times and in how aggressively the disease progresses.

Implications · of prion reproduction

Prions don't trigger immune response, no effective known treatment

Although the discovery of the link between prions and CJD was initially received with great skepticism in the scientific community, subsequent research has supported the conclusion that prions are an entirely new class of infectious pathogens. Furthermore, it is now believed that a similar process of protein malformation may be involved in other, more common degenerative neurological conditions such as Alzheimer's disease and Parkinson's disease. …

Support hypothesis · Corroborating evidence

Further support for prions as a new class of pathogens that reproduce by converting other proteins

This possibility has yet to be fully explored, however, and the exact mechanisms by which prions reproduce themselves and cause cellular destruction have yet to be completely understood.

Limit support for hypothesis · Some unknowns remain

Possible role of prions in other diseases like Alzheimer's and Parkinson's is not yet certain, and questions remain about methods of reproduction and infection

Passage Style

Phenomenon-hypothesis (RC)

Single position

Which one of the following is most strongly supported by the passage?

Understanding the cause of CJD has required scientists to reconsider their traditional beliefs about the causes of infection.

Supported. The traditional belief about infection is that all infections are caused by pathogens that contain nucleic acid. But research showed CJD is caused by a pathogen that doesn’t contain nucleic acid.

93%

CJD is contagious, though not highly so.

Not supported, because the author never discusses the level of contagiousness of CJD.

The prevention of CJD would be most efficiently achieved by the prevention of certain genetic abnormalities.

Not supported, because the author never suggests that CJD is caused by “genetic abnormalities.” Although it is caused by prions that convert their structures into a dangerous abnormal shape, the author doesn’t suggest that this conversion is due to genetic abnormalities.

Although patients with CJD exhibit different incubation times, the disease progresses at about the same rate in all patients once symptoms are manifested.

Not supported, because the author never suggests that CJD progresses at the same rate in all patients once symptoms manifest. Also, the main point isn’t about the timeline of CJD; it’s about the existence of prions as a new class of pathogens.

The prion theory of infection has weak support within the scientific community.

Not supported, because the author doesn’t indicate that the prion theory has only “weak” support. Although it was initially received with skepticism, we know that later research supports the prion theory of infection. In any case, the main point isn’t about the level of support; it’s about the status of prions as a new kind of pathogen.

PrepTest 141 - Section 3 - Passage 1 - Question 2

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