An organism is considered to have an infection when a disease-causing

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
(5) itself in the host organism. 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
(10) widely assumed that all pathogens contain such
genetic material in their cellular structure.

This assumption has been challenged, however,
by scientists seeking to identify the pathogen that
causes Creutzfeldt-Jakob disease (CJD), a degenerative
(15) 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
(20) 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.

(25) 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
(30) 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
(35) 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. Because prions, unlike
(40) 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
(45) themselves, CJD is inevitably fatal, though there are
wide variations in pre-symptomatic incubation times
and in how aggressively the disease progresses.

Although the discovery of the link between prions
and CJD was initially received with great skepticism
(50) 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
(55) common degenerative neurological conditions such
as Alzheimer’s disease and Parkinson’s disease.
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
(60) be completely understood.

1. Which one of the following most accurately expresses the main point of the passage?

(A) Although most organisms are known to produce several kinds of proteins, the mechanism by which isolated protein molecules such as prions reproduce themselves is not yet known in detail.
(B) Research into the cause of CJD has uncovered a deadly class of protein pathogens uniquely capable of reproducing themselves without genetic material.
(C) Recent research suggests that prions may be responsible not only for CJD, but for most other degenerative neurological conditions as well.
(D) The assertion that prions cause CJD has been received with great skepticism in the scientific community because it undermines a firmly entrenched view about the nature of pathogens.
(E) Even though prions contain no genetic material, it has become clear that they are somehow capable of reproducing themselves.

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

(A) Understanding the cause of CJD has required scientists to reconsider their traditional beliefs about the causes of infection.
(B) CJD is contagious, though not highly so.
(C) The prevention of CJD would be most efficiently achieved by the prevention of certain genetic abnormalities.
(D) Although patients with CJD exhibit different incubation times, the disease progresses at about the same rate in all patients once symptoms are manifested.
(E) The prion theory of infection has weak support within the scientific community.

3. 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?

(A) Has the patient suffered a severe blow to the skull recently?
(B) Does the patient experience occasional bouts of insomnia?
(C) Has the patient been exposed to any forms of radiation that have a known tendency to cause certain kinds of genetic damage?
(D) Has any member of the patient’s immediate family ever had a brain disease?
(E) Does the patient’s brain tissue exhibit the presence of any abnormal thread-like structures?

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

(A) The only way in which CJD can be transmitted is through the injection of abnormally shaped prions from an infected individual into an uninfected individual.
(B) Most infectious diseases previously thought to be caused by other pathogens are now thought to bencaused by prions.
(C) If they were unable to reproduce themselves, abnormally shaped prions would not cause CJD.
(D) Alzheimer’s disease and Parkinson’s disease are caused by different conformations of the same prion pathogen that causes CJD.
(E) Prion diseases generally progress more aggressively than diseases caused by other known pathogens.

5. It can be inferred from the passage that the author would be LEAST likely to agree with which one of the following?

(A) The presence of certain abnormally shaped prions in brain tissue is a sign of neurological disease.
(B) Some patients currently infected with CJD will recover from the disease.
(C) Prions do not require nucleic acid for their reproduction.
(D) The body has no natural defense against CJD.
(E) Scientists have only a partial understanding of the mechanism by which prions reproduce

6. Given the manner in which the term “pathogen” is used in the passage, and assuming that the prion theory of infection is correct, which one of the following statements must be false?

(A) Nothing that lacks nucleic acid is a pathogen.
(B) Prions are a relatively newly discovered type of pathogen.
(C) All pathogens can cause infection.
(D) Pathogens contribute in some manner to the occurrence of CJD.
(E) There are other pathogens besides viruses, bacteria, fungi, and parasites.

7. Which one of the following, if true, would most undermine the claim that prions cause CJD?

(A) Several symptoms closely resembling those of CJD have been experienced by patients known to have a specific viral infection.
(B) None of the therapies currently available for treating neurological diseases is designed to block the chain reaction by which abnormal prions are believed to reproduce.
(C) Research undertaken subsequent to the studies on CJD has linked prions to degenerative conditions not affecting the brain or the central nervous system.
(D) Epidemiological studies carried out on a large population have failed to show any hereditary predisposition to CJD.
(E) A newly developed antibacterial drug currently undergoing clinical trials is proving to be effective in reversing the onset of CJD.