PT141.S3.P1.Q3 > Analytics

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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

If the hypothesis that CJD is caused by prions is correct, finding the answer to which one of the following questions would tend most to help a physician in deciding whether a patient has CJD?

Question Type

Application

The correct answer is likely to be supported by P3, which describes how prions work to damage nerve cells in the brain. A physician would probably want to know whether any of the things described in P3 are occurring in order to determine whether the patient has CJD.

Has the patient suffered a severe blow to the skull recently?

We have no reason to think that external damage to the brain has any role to play in the cause of CJD.

Does the patient experience occasional bouts of insomnia?

Although “chronic” insomnia is a symptom of CJD, determining whether there are “occasional” bouts of insomnia doesn’t help determine whether someone has CJD. Chronic insomnia is, by definition, not just an occasional bout of insomnia.

Has the patient been exposed to any forms of radiation that have a known tendency to cause certain kinds of genetic damage?

We have no reason to think genetic damage has anything to do with the cause of CJD.

Has any member of the patient's immediate family ever had a brain disease?

We have no reason to think that genetics have anything to do with CJD. So whether a family member has a brain disease provides no signal to a doctor about whether another family member has CJD.

Does the patient's brain tissue exhibit the presence of any abnormal thread-like structures?

An abnormal thread-like structure of plaque in the brain destroys brain cells. So it would help a doctor determine whether someone has CJD if she could learn whether the person has abnormal thread-like structures in the brain. If the person doesn’t, CJD is unlikely to be present. If the person does, then that’s a sign of potential CJD.

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