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MCAT Sample Questions : Biological Sciences

Passage III

The runny nose associated with common colds is related to the release of either virus-induced histamine or acetylcholine. The nasal mucosa contain receptors for both histamine and acetylcholine. Activation of either of these receptor types results in increased secretion by nasal glands, producing a runny nose.

In an attempt to treat this condition, several new drugs have been investigated. Drug A primarily blocks histamine receptors but also partially blocks acetylcholine receptors. Drug B blocks histamine receptors but has no effect on the acetylcholine receptors.

Eighteen subjects with severe common colds, whose symptoms were judged to be identical, were randomly assigned to three groups. Patients A-F in Group 1 received Drug A, Patients A-F in Group 2 received Drug B, and those in Group 3 were treated with a nondrug placebo. After 4 hours, patients reported their own runny nose symptoms on a 5-point scale ranging from a dry nose (1) to an excessively runny nose (5).

Table 1 Self-Reported Symptoms 4 Hours after Treatment

Subject Group 1 (Drug A) Group 2 (Drug B) Group 3 (Placebo)
A 2 3 1
B 1 4 4
C 3 2 5
D 2 3 4
E 1 3 5
F 1 3 4

Following are some sample questions on this passage:

  1. In Group 3, the response of Patient A can best be classified as a response most likely:

    1. not associated with the treatment.
    2. associated with histamine blocking only.
    3. associated with acetylcholine blocking only.
    4. associated with a combination of histamine and acetylcholine blocking.

    Answer: A

    Explanation: Patient A in group 3 received a placebo, and so was untreated. Nevertheless, a response was observed, a reduction in symptoms (lower rating on the 5-point scale). The correct answer, therefore, is choice A, that the response was one not associated with treatment. The other answer choices, B, C and D, would only be plausible if the patient in question had received treatment with a histamine blocker, an acetylcholine blocker or drug with both properties. Thus, answer choice A is the best answer

  2. Based on the passage, which drug treatment would hypothetically provide the maximum reduction in nasal secretions?

    1. Antihistamine only
    2. Acetylcholine only
    3. Antihistamine and acetylcholine
    4. Antihistamine and acetylcholinesterase

    Answer: D

    Explanation: Drug A blocks histamine receptors and acetylcholine receptors. The group that received Drug A reported the lowest level of symptoms. This suggests that decreases in the effect of both histamine and acetylcholine should be the optimum treatment. Answer choice D represents this combination. The histamine component of the symptom reduction is achieved by blocking its histamine receptors with an antihistamine. Introducing acetylcholinesterase, an enzyme that catalyzes the degradation of acetylcholine, reduces the acetylcholine component. The question assumes that the examinee knows that acetylcholinesterase breaks down acetylcholine or at least recognizes that the term refers to an enzyme for which acetylcholine is a substrate. Thus, answer choice D is the best answer.

  3. Based on Table 1, the individual who benefited most from a specific blocking effect on the histamine receptors only is:

    1. Subject B in Group 1.
    2. Subject C in Group 1.
    3. Subject B in Group 2.
    4. Subject C in Group 2.

    Answer: D

    Explanation: The passage states that drug B blocks histamine receptors only. Group 2 received Drug B. Of all the subjects in group 2, subject C reported the lowest level of symptoms. Answer choice D is therefore the correct response: that the person who received the most benefit from blocking of histamine receptors alone was subject C in group 2.

  4. The nasal mucosa cells responsible for the release of excessive fluid during the common cold can best be classified as:

    1. epithelial.
    2. connective.
    3. contractile.
    4. neurosecretory.

    Answer: A

    Explanation: Epithelial cells cover all the free surfaces of the body forming an interface between body tissues and the external environment. When fluid is released from the body it must cross an epithelial surface. Answer choice A is therefore the correct choice. The cells that secrete mucous are primarily found in mucosal glands made up of ectodermal epithelium continuous with the epithelial lining of the nasal cavities. None of the other types of tissue named, connective (answer B), contractile (answer C) or neurosecretory (answer D) impinge on the external environment.

  5. An acetylcholinesterase inhibitor increases nasal secretions because it:

    1. blocks acetylcholine release from parasympathetic nerve endings.
    2. blocks acetylcholine response at acetylcholine receptors.
    3. increases parasympathetic activity at acetylcholine receptors.
    4. decreases parasympathetic activity at acetylcholine receptors

    Answer: C

    Explanation: Acetylcholinesterase is an enzyme that catalyzes the degradation of the neurotransmitter acetylcholine into choline and acetic acid. An acetylcholinesterase inhibitor prevents the breakdown of acetylcholine. This causes acetylcholine to remain in the synaptic cleft for a longer period of time, stimulating the nasal epithelial cells to secrete. The correct response to the question is therefore answer choice C, that acetylcholinesterase inhibitors must increase parasympathetic activity at the acetylcholine receptors. Answer choice D states the opposite, that it decreases activity. Answer choices A and B are incorrect because they describe alternatives that would decrease rather than increase synaptic conduction.

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