Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 571:

  Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

  These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

  Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.

  The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.

  How might one most practically assess the risk of erythroblastosis fetalis in a pregnant woman?

  A. Test all women for the presence of anti-Rh antibodies.

  B. Test all fetuses for the presence of the Rh antigen within the first trimester of pregnancy.

  C. Test only Rh?mothers for the presence of anti- Rh antibodies.

  D. Test all mothers of Rh+ children for the presence of anti-Rh antibodies.

  Correct Answer: C

  In answering this question, you must have an understanding of Rh incompatibility and also keep in mind that you're asked to choose the most PRACTICAL screening program. Rh incompatibility exists when an Rh negative mother, who has been sensitized to the Rh antigen by a previous pregnancy, is pregnant with another Rh positive baby. The risk exists in that the mother, as a result of this previous exposure to the Rh antigen, has produced antibodies that will now attack the fetus' red blood cells, which are Rh positive. So, the first step in making the assessment of erythroblastosis fetalis risk is to narrow down the group being assessed. Well, the mother must definitely be Rh negative for there to be ny risk at all, which means that testing all women, as a choice A, is extremely impractical. So choice A is incorrect. Choice B is incorrect because it tests all fetuses for the presence of the Rh antigen. This is useless unless the mother is known to be Rh negative. The most practical test is the one that singles our Rh negative mothers for testing. So, choice B is also incorrect. Choice C, however, is practical. It does single out Rh negative mothers to test them for the presence of anti-Rh antibodies. Choice D is not practical because all mothers of Rh positive children, regardless of their own blood type, are tested. Both Rh positive and Rh negative mothers would be tested, which is unnecessary and expensive. So, choice C is the correct answer; test Rh negative mothers to determine whether or not they are producing anti-Rh antibodies. If the test is positive, then there is a risk of erythroblastosis fetalis for an Rh positive fetus.

• Question 572:

  Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

  These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

  Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.

  The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.

  If a man with type AB blood needed a transfusion of red blood cells, which of the following individuals could safely donate blood?

  A. A man with type A blood

  B. A man with the genotype BO

  C. A woman with the genotype AB

  D. All four blood types are equally safe

  Correct Answer: D

  The thinking process behind this question is similar to the one used to answer the previous question. A person with type AB blood expresses both the A and B antigens on his red blood cells, which implies that his blood does NOT contain any anti-A or anti-B antibodies. Since the recipient's blood does not contain anti-A antibodies or anti-B antibodies; this means that any blood type can b safely transfused, regardless of the A and B antigens found in the donor's blood. Be aware that there are other blood antigens typically present that could cause problems during transfusions, but this is beyond the scope of this question. Also recognize that the gender of the person donating the blood is in no way relevant.

• Question 573:

  Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

  These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

  Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk. The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.

  A new virus has been discovered that evades detection by the immune system of only those individuals with type A or type AB blood. Which of the following best accounts for this observation?

  A. The viral antigens resemble the A antigen.

  B. The viral antigens resemble the B antigen.

  C. The viral antigens are

  D. The viral antigens are too small to elicit an immune response.

  Correct Answer: A

  From this question stem you know that this virus avoids detection by the immune system of only those individuals who have type A or AB blood. From this fact you can conclude that the virus must NOT evade the immune system of people with B or O blood types. So the correct answer will differentiate between these two groups. Well, from this piece of information you can eliminate choice D. If the viral antigens are too small to elicit an immune response, the virus would evade all immune systems, not just those of individuals with A or AB blood types. Choice C can also be eliminated since the presence of the Rh factor is independent of ABO blood type. So now we need to decide between choices A and B. How do type A and type AB blood differ from type B and type O blood. Well, individuals with type A and AB blood express the A antigen on the surface of their red blood cells. These individuals will therefore NOT produce antibodies to the A antigen. On the other hand, individuals with type B and O blood do NOT express the A antigen, thus they will produce antibodies to the A antigen. Well if the virus' antigen mimicked the A antigen, then people who normally express the A antigen -- that is, people with type A or type AB blood -- would not recognize the virus as foreign. Thus, no immune response would be elicited, and the virus would be able to persist in these people. Thus, choice A is the correct answer. If the viral antigens mimicked the B antigen, as in choice B, then the virus would evade detection by the immune system in those individuals who normally expressed the B antigen -- type B and type AB people, but would be detected by the immune system of those individuals who do not normally express the B antigen, namely, type A and type O people.

• Question 574:

  Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

  These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens

  on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

  Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.

  The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.

  A couple decide to have a child. If the father's genotype is AO and the mother has type B blood of unknown genotype, which of the following are possible blood types for their child?

  I. A

  II. B

  III. A, B

  IV.

  O

  A.

  I and II only

  B.

  I, II, and III only

  C.

  I, II, and IV only

  D.

  I, II, III, and IV

  Correct Answer: D

  This is one of those questions requiring an understanding of simple genetics and the ABO system. If you didn't already know it, you're told in the passage that the A and B alleles are codominant to each other, and that the O allele is recessive. Codominance means that both the alleles are phenotypically expressed. So, when a person has both the A and the B alleles, a person is said to have type AB blood and expresses the properties ascribed to BOTH alleles -- that is, their red blood cells have both the A and B antigens on their surface. During sexual reproduction, the mother and father each donate one allele to their offspring. In this case we know the father's genotype is AO. This means that he can donate either an A allele or an O allele to his child. We don't, HOWEVER, know the mother's genotype; we only know that her phenotype is type B blood. Well, this means that her genotype is either BO or BB -- we simply don't know which one it actually is. Let's first assume the mother to have the genotype BB and must therefore donate a B allele to the child. In this situation, if the father donates an A, the child's genotype and phenotype will b ABk. If the father donates an O allele, the child's genotype will be BO and the phenotype will be type B. Therefore, statements II and III are correct. Well, since III is correct, you can rule out choices A and C because they don't contain it. Now let's assume the mother's genotype to be BO. This means that she can donate an O allele to the child. In this case, if the father donates an A allele, the child's genotype will be AO and the phenotype will be A. This means that statement I is also correct; but this doesn't help you decide between choices B and D because they both contain statement I. In fact, you should have known that I was correct because it appears in both of these remaining choices. So what it comes down to is whether or not this child could have type O blood. Well, if the father donates an O allele, the child's genotype will be OO and the phenotype could have type O. This means that statement IV is also correct; all four blood types are possibilities.

• Question 575:

  The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction.

  

  Which of the following alkyl halides would be most likely to react with sodium butanoate to form an ester?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  In this question, you are asked to decide which alkyl halide would be most likely to react with sodium butanoate and form an ester. The reaction between a carboxylate anion and an alkyl halide is an SN2 reaction. Because this reaction involves partial bonding between the attacking nucleophile (carboxylate) and substrate (alkyl halide), it will not occur easily with bulky substrates. Bulky substrates sterically hinder the reaction, by shielding the susceptible carbon atom from the attacking nucleophile. Choice B, methyl chloride, has only hydrogen and chlorine substituents, whereas all the other choices have bulkier alkyl substituents. This makes B the least sterically hindered of all the choices, and it will therefore react most easily with sodium butanoate.

• Question 576:

  Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

  These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

  Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.

  The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies. In a paternity case, the mother has type A+ blood and her son has type O- blood. If the husband has type B+ blood, which of the following is true?

  A. The husband could be the father.

  B. The husband could not be the father.

  C. The husband could not be the father of a son, but could be the father of a daughter.

  D. The husband is definitely the father.

  Correct Answer: A

  Let's determine as much as we can about the blood types of the parents and the son. We know that the mother is blood type A positive, and therefore one of her alleles codes for the A antigen. We don't know whether the other allele for that locus is an O or A. The mother is also Rh positive, and again, similar reasoning applies here. We know that she has at least one Rh allele that codes for the Rh antigen, which makes her Rh positive, since the Rh allele is dominant. We don't know about the second allele at that locus; it might be either Rh positive of Rh negative. Let's take a look at the son. The son is O negative, so here we know his genotype precisely. His genotype for the ABO blood groups is OO. Likewise, with respect to the Rh locus, he's doubly negative. If he had just one Rh allele he would be Rh positive, since the allele is dominant. So the son's genotype must be OO Rh negative Rh negative. Working backwards, we can deduce that his mother's genotype must be AO Rh positive Rh negative, since the son inherited one allele per locus from his mother. Let's take a look at the husband. The husband is type B positive, so we know that he has to have at least one B allele and one Rh allele. In the paternity case the question to be addressed is, is it possible for the husband to be the father of this boy, knowing what we know about the mother? The answer is yes, it's possible. We know that the mother's genotype is AO Rh positive, Rh negative; now suppose that the husband's blood type is BO Rh positive Rh negative. If this were the case, then the son could have inherited one O allele from each parent and one Rh negative allele from each parent. Therefore, it is possible for these two people to conceive an O negative child. thus, the correct answer is choice A; the husband could have been the father. We can't answer D, that the husband was definitely the father, because we just don't know; maybe the mother had an affair with another man whose blood type contained both the O allele and the Rh negative allele. From our discussion, choices B and C are obviously incorrect. And besides, the gender of the child has no influence whatsoever on blood type.

• Question 577:

  The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction.

  

  Esterification may also occur between parts of the same molecule. Which of the following compounds would most easily undergo internal esterification to form a cyclic ester?

  

  A. Option A

  B. Option B

  C. option C

  D. Option D

  Correct Answer: C

  This question asks which compound would most easily undergo internal esterification to form a cyclic ester. In this case, the stability of ring structures must be considered. The least amount of ring strain occurs in structures that can form six-membered rings, like cyclohexane. If choice A were to form a cyclic ester, it would be a four-membered ring with a lot of angle and eclipsing strain. Choice B would be a five- membered ring, which is not ideally stable. Choice C, if it underwent internal esterification, would form a six-membered ring. Five of the atoms comprising the ring would be carbons, and one would be an oxygen. This formation is strain free; therefore, it would be most easily formed. Choice D would yield a seven- membered ring, which is less stable than choice C.

• Question 578:

  The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction.

  

  Another method for forming esters is:

  

  Why does this reaction occur?

  A. The halide is a poor leaving group.

  B. The halide acts as a good nucleophile.

  C. The halide is an electron-donating group.

  D. The carboxylate anion is highly nucleophilic.

  Correct Answer: D

  This question asks you to explain why a reaction occurs between carboxylic acids and alkyl halides. This reaction is an example of a nucleophilic substitution reaction with the carboxylate ion serving as the nucleophile and the leaving group. On the basis of this information, choice D is the correct response. Choice A is wrong because, in this case, the halide is a very good leaving group. Choice B is wrong because the halide is not the nucleophile but the leaving group. And finally, choice C is incorrect because the leaving group is an electron withdrawing group by virtue of its electronegativity.

• Question 579:

  The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction.

  

  Assuming that only the forward reaction occurs, which of the following statements is correct?

  A. The ester will contain labeled oxygen, while the water fraction will not.

  B. The water fraction will contain labeled oxygen, while the ester will not.

  C. Both the water fraction and the ester will contain labeled oxygen.

  D. The location of the labeled oxygen cannot be determined.

  Correct Answer: A

  This question concerns the acid-catalyzed esterification of a carboxylic acid. In this reaction, the carbonyl carbon of the carboxylic acid reacts with the alcohol oxygen to form an ester linkage. Therefore, if the alcohol has a tagged oxygen, the ester itself will be tagged when the reaction is complete. Thus Choice A is correct. Choices B, and C are incorrect because the water fraction will not contain labeled oxygen, since the oxygen of the water comes from the hydroxyl group on the carboxylic acid, which was never labeled. The answer CAN be determined from knowledge of the mechanism, so choice D is incorrect as well.

• Question 580:

  The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction.

  

  The rate of the reaction is negligible without the acid catalyst. The catalyst:

  A. attacks the carbonyl oxygen, permitting the nucleophilic group to attack the carbonyl carbon.

  B. attacks the carbonyl carbon, permitting the nucleophilic group to attack the carbonyl oxygen.

  C. attacks the carbonyl oxygen, permitting the electrophilic group to attack the carbonyl carbon.

  D. attacks the carbonyl carbon, permitting the electrophilic group to attack the carbonyl oxygen

  Correct Answer: A

  This question asks why the esterification of a carboxylic acid with an alcohol should be carried out under acid catalysis. To answer this, it's important to understand the mechanism presented in the passage. In the first step, the proton of the acid catalyst attacks the oxygen of the carboxylic acid carbonyl group. This produces a positively charged carbon, which is very susceptible to nucleophilic attack by the alcohol. Thus the reaction occurs between the carbonyl carbon, which is electrophilic and the alcohol oxygen, which is nucleophilic. Choice A agrees with this mechanism and is therefore the correct response.