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MCAT Sample Questions : Biological Sciences

Passage IV

Cholesterol is a structural component of plasma membranes and a precursor of steroid hormones. The typical blood cholesterol level of healthy humans is about 1.8 mg/mL.

Familial hypercholesterolemia (HC) is a disease associated with high cholesterol levels; HC affects 1 in 500 people. The cholesterol level of moderately affected individuals is about 3.0 mg/mL. Severely affected individuals have cholesterol levels around 7.0 mg/mL.

HC is relatively common in some families and absent from others. Research shows no significant difference between the dietary habits of individuals in affected and unaffected families.

Biologists believe that HC is caused by a malfunction at the plasma membrane. The cells of healthy individuals have cholesterol-containing endocytotic vesicles, whereas the cells of individuals with HC do not. A comparison of the homogenized cells reveals that individuals with HC lack a membrane protein that normally binds to a particular cholesterol-containing particle (LDL). This same protein is found in the membrane of cholesterol-containing vesicles.

Following are some sample questions on this passage:

  1. Cholesterol is a precursor of which of the following hormones?

    1. Insulin
    2. Gastrin
    3. Thyroxin
    4. Estrogen

    Answer: D

    Explanation: The passage states that cholesterol is a precursor of steroid hormones. Of the compounds listed, estrogen is a steroid hormone. Therefore, the key is D.

  2. Is it reasonable to conclude that HC is caused by a genetic disorder?

    1. Yes, because HC is common in certain families, regardless of the type of diet they consume.
    2. Yes, because HC is common in families that consume low-cholesterol diets.
    3. No, because individuals in families that consume high levels of cholesterol are more likely to acquire the disease.
    4. No, because HC appears to be caused by a defective protein and not a defective DNA sequence.

    Answer: A

    Explanation: Evidence that familial hypocholesterolemia (HC) is caused by a genetic disorder comes from two facts: (1) it is relatively common in some families and absent in others, and (2) it develops independent of diet. This combination is described in choice A. Answer choices B and C imply that it is related to diet. Answer choice D suggests that this consistent familial protein defect might be independent of a DNA defect. Because DNA codes protein structure and DNA is the molecular basis of heredity, this cannot be a correct choice. Therefore, answer choice A is the best answer.

  3. Researchers can most effectively mimic the symptoms of HC in a healthy person if they give that person a drug that:

    1. enhances the absorption of lipids from the small intestine.
    2. binds to the plasma membrane proteins that normally bind LDL.
    3. inhibits both the production and the secretion of bile.
    4. prevents the synthesis of all plasma membrane proteins.

    Answer: B

    Explanation: The cause of HC presented in the passage is the inability of membranes of affected individuals to bind the blood-protein that transports cholesterol, the low-density lipid binding protein, LDL. The drug described in answer choice B would be the best model of HC. Such a drug could, by preventing membrane binding of LDL, disrupt normal functioning in the same way as proposed for HC. Increasing absorption of lipids by the small intestine (answer A) would not mimic HC as well; it would just increase all types of lipids in the blood. Lowering the bile in the small intestine (answer C) would prevent lipid emulsification, hence absorption, but would not be a good model. Decreasing plasma membrane protein production (answer D) would not be a good model because it would produce widespread effects on the body that would be difficult to untangle from the effect of loss of membrane binding proteins alone. Thus, answer choice B is the best answer.

  4. A man and a woman, each with a cholesterol level of about 3.0 mg/mL, have a child with a cholesterol level of 7.0 mg/mL. Assuming that HC is determined by alleles at a single locus, does this observation provide evidence that the HC allele is dominant to the normal allele?

    1. No; HC is recessive, because the disease is expressed in the child but is not expressed in the parents.
    2. No; HC is codominant, because the heterozygous parents have a less severe form of the disease than does the homozygous child.
    3. Yes; HC is completely dominant, because the child inherited the most severe form of the disease.
    4. Yes; HC is completely dominant, because both parents carry the allele and have the disease.

    Answer: B

    Explanation: Both the father and mother have cholesterol levels of 3.0 mg/mL, which is in the moderately affected range. This suggests they have one copy of the HC gene and one copy of the normal (wild-type) gene and that the two versions of the gene (alleles) are co-dominant. When a gene is recessive, its effect (or lack of effect) is masked by a corresponding dominant gene. The child has a cholesterol level of 7.0 mg/mL. This is in the severely affected range according to the passage. This child appears to have acquired one HC gene from each parent for a total complement of two HC genes and no normal genes. This is consistent with the idea that the normal and HC genes are co-dominant. Therefore, the best answer is answer choice B.

  5. Vasoconstriction of which of the following vessels will most effectively reduce fat absorption from the small intestine into the bloodstream?

    1. Lacteals inside intestinal villi of the small intestine
    2. Capillaries in the smooth muscle of the small intestine
    3. Lacteals in the peritoneum around the small intestine
    4. Capillaries in the peritoneum around the small intestine

    Answer: A

    Explanation: If the lacteals of the intestinal villi were to contract, it would reduce fat absorption because ingested fats collect in the lacteals for transport to the venous (portal) circulation. The correct answer therefore is answer choice A. If the capillaries of the intestinal smooth muscle constricted, the rhythmic contractions of the intestines would be reduced, but there would be no specific effect on fat absorption. Any constriction of blood vessels associated with the peritoneal covering of the intestine would have no effect on fat absorption, because this covering is on the outside of the organ, opposite the luminal surface where fat absorption takes place.

  6. Which of the following statements provides the strongest support for the hypothesis that HC is a genetic disease rather than a disease caused by environmental factors?

    1. HC is relatively common in some families and absent from others.
    2. There is no significant difference in the dietary habits of individuals in affected families versus individuals in unaffected families.
    3. HC appears to be caused by a malfunction at the plasma membrane.
    4. The cells of healthy individuals have cholesterol-containing endocytotic vesicles, whereas the cells of individuals with HC do not.

    Answer: B

    Explanation: The strongest support for the hypothesis that HC has a genetic rather than an environmental origin comes from the fact that it develops independent of diet. Answer choice B is the correct statement: that HC develops when diet is held steady. Answer choice A does not address the question of whether affected and unaffected families have different diets. Answer choices C and D address issues that do not speak to the question of whether the disorder is genetic or environmental.

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