MCAT Biology : Other Hormone Physiology

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Example Question #154 : Endocrine System

There are a large variety of over-the-counter pregnancy tests, although all of them share a set of common principles.

These home pregnancy tests are immunoassays which detect the presence of the peptide hormone human chorionic gonadotropin (hCG). hCG is produced by the fertilized ovum and released into the mother's bloodstream, preventing the degradation of the corpus luteum and thereby preventing menstruation. By the time of the mother's first missed period, hCG levels in the urine are high enough to detect.

The home pregnancy test generally involves dipping a stick containing immobilized hCG monoclonal antibodies into a urine sample. If hCG is present in the urine, it will bind to the monoclonal antibodies on the dipstick. The dipstick is then placed in a solution containing a second monoclonal antibody which recognizes the bound hCG antibody complex on the surface of the dipstick. This second antibody is conjugated to colloidal gold particles which change color when they are immobilized, indicating a positive test result (i.e., pregnancy).

The same techniques used in the home pregnancy test can be implemented in other clinical tests. Which of the following could not be detected using such techniques?

Possible Answers:

Tryptophan blood levels

Autoimmune diseases such as rheumatoid arthritis

Reiter's Syndrome caused by a certain strains of bacteria

Strep throat caused by a particular strain of bacteria

HIV infection 

Correct answer:

Tryptophan blood levels


Answering this question simply requires picking the answer choice that doesn't have anything in common with the others. Conditions in which specific antibodies are produced which might be tested for using the same technique are described in the passage. After all, antibodies are proteins, and monoclonal antibodies can be made to detect them. HIV infection can be detected by assaying for the antibodies against HIV.Arthritis can be detected by looking for antibodies against "self" proteins. Strep throat can be detected by directly assaying for the presence of bacterial proteins. The same technique which was used to detect hCG can be used in these instances. Tryptophan blood levels, however, cannot be determined using the same methodology. For one thing, tryptophan (an amino acid) does not elicit antibody production. Secondly, the actual levels of substances in the blood can't be detected by the type of test described in the passage. This test yields either a "present" or "not-present" result

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