Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 51:

  

  Which of the following expressions best approximates the work required to move the box with a mass m to the top of the ramp? (Assume that the ramp is frictionless.)

  A. mglcosT

  B. mghsinT

  C. mgdcosT

  D. mgdsinT

  Correct Answer: D

  Explanation: This question asks the examinee to determine the work required to move a box from the bottom of a ramp to the top of a ramp. While a ramp provides a mechanic advantage, which reduces the force required to push the box to the top of the ramp, the work required to move the box using the ramp is the same amount of work required to lift the box to a height h without the use of a ramp. Therefore, the correct answer is mgdsinT, which is equal to mgh.

• Question 52:

  A source emits a sound from one medium with a certain velocity, intensity, frequency and wavelength. When the sound exits the first medium and enters a denser medium, all of the following changes EXCEPT:

  A. velocity

  B. intensity

  C. frequency

  D. wavelength

  Correct Answer: C

  This question is testing your understanding of the properties of sound as waves travel from one medium to another. To answer this question, you must know that the frequency of sound and light always remains constant when changing mediums. The wavelength on the other hand does change, and a change in the wavelength causes a proportional change in the velocity since v = f. Intensity is also proportional to velocity and so A, B, and D are all incorrect. Thus, C is the correct answer.

• Question 53:

  Arginine is one of the 20 most common natural amino acids. Most healthy people do not need to supplement with arginine because the body usually produces sufficient quantities. The pathway for arginine synthesis was studied using cells from a red bread mold. This natural form of arginine is illustrated below.

  

  The red bread mold Neurospora crassa grows well on a cultural plate with "minimal" medium which is a fluid containing only a few simple sugars, inorganic salts, and vitamin. Neurospora that grows normally in nature (wild type) has enzymes that convert these simple substances into the amino acids necessary for growth. Mutating any one of the genes that makes an enzyme can produce a Neurospora strain that cannot grow on minimal medium. The mutant would only grow if the enzyme product were to be added as a supplement. On the other hand, if a "complete" medium is provided, containing all required amino acids, then Neurospora would grow, with or without mutation.

  

  Figure 1 A synthesis pathway for the amino acid arginine. Each gene in italics in the diagram produces one enzyme necessary for the synthesis of this essential amino acid required for growth.

  

  Table 1 Growth response of mutant strains in "minimal" media with supplements (ornithine, citrulline, argininosuccinate, and arginine) as indicated. Strain growth is indicated by (+) and no strain growth is indicated by (-). Which of the following is NOT an accurate description of naturally-occurring arginine?

  A. Acidic amino acid

  B. L-Configuration

  C. -Amino acid

  D. S-Configuration

  Correct Answer: A

  The labeling of amino acids as acid, basic, hydrophilic, hydrophobic is based on the side chain. Two amino acids have acidic side chains at neutral pH. These are aspartic acid or aspartate (Asp) and glutamic acid or glutamate (Glu). Their side chains have carboxylic acid groups whose pKa's are low enough to lose protons, becoming negatively charged.

  Arginine and lysine are hydrophilic, basic amino acids. Their pKa's are high enough that they tend to bind protons, gaining a positive charge (note that nitrogen can accept a proton). Histidine can act as an acid or base thus it is a good physiologic buffer.

  Naturally occurring amino acids are -amino acids with the L-configuration (as opposed to most naturally occurring carbohydrates which are D-configuration).

  The S-configuration can be confirmed by assigning priority at the carbon (i.e. carbon-2 which is the neighbor of the carbonyl C=O carbon): N > COOH > CC which is counterclockwise. Note that it is implied that the lowest group must be a hydrogen pointing away from the viewer since the dark triangle represents a group pointing out of the plane.

• Question 54:

  In an SDS-PAGE procedure, the SDS serves as a detergent.

  Why are the proteins treated with a detergent before being run through the electrophoresis gel?

  A. To coat the proteins with a large positive charge, since amino acid side chains may have positive, negative, or neutral charges, and a large uniform charge is necessary to get good separation in the gel.

  B. To allow the electrophoresis to separate the proteins solely on the basis of the length of the primary sequence.

  C. To prevent the protein from denaturing so that the electrophoresis can accurately resolve the proteins on the basis of tertiary structure.

  D. To break the intramolecular bonds holding the tertiary and primary structure of the protein together, thereby generating linear fragments that may be sorted on size.

  Correct Answer: B

  SDS is a detergent which denatures the tertiary and secondary structure of a protein. It also coats the protein with a very large negative charge. This electrostatic repulsion pushes the protein in a single long rod shape, allowing the gel to sort various proteins on the basis of primary structure length. Thus (B) is the right answer.

  A: This answer choice is right except is says positive charge. SDS creates a negative charge.

  C: Detergents cause denaturing, rather than preventing it.

  D: SDS will break down tertiary and secondary structure, not primary.

• Question 55:

  Triacylglycerides consist of:

  I) a ribose backbone II) a glycerol backbone III) three phosphodiester linkages IV) three ester linkages

  A. I and III

  B. II only

  C. II and III

  D. II and IV

  Correct Answer: D

  Triacylglycerides consist of a glycerol backbone bonded to three fatty acids. The bond between each acid and the glycerol is an ester group. Phosphodiester linkages are found in DNA.

  A ?incorrect. Glycerol, not ribose, forms the backbone of triacylglycerides.

  B ?incorrect. There are three ester linkages in addition to the glycerol backbone.

  C ?incorrect. Phosphodiester linkages are found in DNA not triacylglycerides.

• Question 56:

  Which of the following molecules is a hormone capable of creating physiological and anatomical changes during adolescence which will last a person's entire life?

  A.

  B.

  C.

  D.

  Correct Answer: B

  A: Polypeptide. Note all of the amide bonds. This polypeptide is vasopressin.

  B: Classic four-ring steroid structure. This is estradiol.

  C: Sugar ring attached to nitrogenous base makes this a nucleotide. This is cytosine monophosphate.

  D: Lengthy carbon chains attached to a phosphate group indicates a phospholipid.

• Question 57:

  The combination of the cornea and the crystalline lens shown below serves as a converging lens.

  

  In a perfectly functioning eye, an image is projected at a fixed distance on the retina, which is approximately 2cm from the lens. What is the power of the converging lens when a person without any visual impairment stares at a distant object?

  A. 0.5 diopters

  B. 10 diopters

  C. 25 diopters

  D. 50 diopters

  Correct Answer: D

  To answer this question, you must use the thin lens equation 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance. Since the power (P) of a lens is equal to 1/f, you can set P = 1/do + 1/di. The question stem states that image distance is fixed and equal to a length of 2 cm. Since the question states that the observer stares at a distant object, we can assume that 1/do is negligible and equal to zero. Therefore, the equation simplifies to P = 1/di. Since power is measured in diopters, which has the units of m-1, you must convert 2 cm to 0.02 m and solve. Thus, the power of the lens is equal to 50 diopters, or answer choice D.

• Question 58:

  Assuming the circulatory system in humans obeys Bernoulli's principle of fluid dynamics, which of the statements most accurately compares the blood pressure in a capillary of the neck to a capillary with an equal cross-sectional area in the right knee?

  A. The pressure in the neck is greater than the pressure in the knee because of the increase in pressure head.

  B. The pressure in the neck is equal to the pressure in the knee because of the equal dynamic pressure according to the continuity equation.

  C. The pressure in the knee is greater than the pressure in the neck because of the increase in pressure head.

  D. An accurate comparison cannot be given without knowledge of the fluid's density and viscosity.

  Correct Answer: C

  This question is asking the examinee to compare the blood pressure in the capillary beds in two different regions of the body. To answer this question, you must apply Bernoulli's equation ?P + 1/2pv2 + pgh ?at a point in the neck and a point in the knee. Since the cross sectional area is equal, the fluid velocity will also be equal. Thus, Bernoulli's equation can be simplified as shown:

  Pneck + pghneck = Pknee + pghknee

  Since hneck is greater than the hknee, the pressure in the neck is lower than the pressure in the knee. Thus, the correct answer is choice C.

• Question 59:

  For a very weak base, the pKb of a solution would likely be:

  A. Equal to the pOH

  B. Higher than the pOH

  C. Lower than the pOH

  D. Near 7 at 25篊

  Correct Answer: B

  The correct answer is B. Normally to calculate pH, pOH, and pKa values, we require some numerical information about the concentration of the solution. In this question that is not given so we will have to go off of the concepts alone. The

  MCAT often does this and the question is still very doable, so do not panic when you find numbers are not provided. Let us write a generic base reaction to represent our molecule:

  B + H2O <----> HB+ + OH- Kb = [OH-][HB+]/[B]

  Using an alternate form of the Henderson-Hasselbalch Equation, expressed in terms of pOH and pKb:

  pOH = pKb + log [HB+]/[B],

  where [HB+] and [B] represent the concentrations of the conjugate acid and original base molecule at equilibrium, respectively.

  A very weak acid will have a very small Kb.This means that equilibrium favors the reactants [B] over the products [HB+]. Thus, we can infer that the value of [HB+]/[B] << 1, making the log [HB+]/[[B] << 0. This would result in a pOH value that is less than the pKb. Thus, pOH < pKb.

• Question 60:

  A thin layer chromatography is performed on both the reactants and products of a reaction. It is found that the products have an Rf value that is significantly higher than the reactants.

  Which of the following could adequately describe this reaction?

  A. SN2 reaction converting an alkyl bromide to an alkyl chloride

  B. Addition reaction converting an alkene to an alcohol

  C. Nucleophilic acyl substitution reaction converting an ester to an anhydride

  D. Elimination reaction converting an alcohol to an alkene

  Correct Answer: D

  In a typical TLC, a high Rf value indicates a nonpolar molecule that moves with the solvent and a low Rf value indicates a polar molecule that sticks to the silicate of the glass slide. The problem describes products with a higher Rf value so we're looking for a reaction that goes polar --> less polar. Choice (D) is the correct answer as it shows a highly polar molecule, an alcohol, being converted to a less polar molecule, an alkene. The remaining choices:

  A: polar ? polar

  B: less polar ? more polar

  C: polar ? slightly more polar