Medical Tests MCAT-TEST Online Practice Questions and Exam Preparation

Medical Tests MCAT-TEST Online Questions & Answers

• Question 751:

  The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.

  When the battery is being discharged, the following reaction takes place:

  

  Reaction 1

  The electrode reactions, both written as reductions, are shown in Table 1.

  Table 1

  Half-reaction

  E?V)

  PbO2(s) + SO42-(aq) + 4H+(aq) + 2ePbSO4(s) + 2H2O

  PbSO4(s) + 2e-Pb(s) + SO42-(aq)

  1.69

  ?.36

  As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.

  Often in cold weather the battery goes "dead". Thermodynamic data confirms that the voltage of most electrochemical cells decreases with decreasing temperature. If the battery is warmed to room temperature, it often recovers its ability to deliver normal power. The battery appeared "dead" because:

  I) the resistance of the electrolyte had decreased. II) the viscosity of the electrolyte had increased. III) the viscosity of the electrolyte had decreased.

  A. I only
  B. II only
  C. I and II only
  D. I and III only
  B. II only

  ---

  Explanation/Reference:

  The reason that the battery "goes dead" with decreasing temperature is that the viscosity of the electrolyte increases. At higher viscosities the ions are moving much slower, which leads to an increase in resistance and a decrease in the power output. Roman numeral I states that the resistance of the electrolyte has decreased. This is not true, the resistance of the electrolyte increases with decreasing temperature. Since choice A, choice C, and choice D all contain Roman numeral I, they can be eliminated, leaving choice B as the correct answer.

• Question 752:

  Breast milk is proven to be the best form of nutrition for infants and is recommended for at least 1 year after birth. Unfortunately, a recent survey of breastfeeding mothers revealed that only 20% continue breastfeeding or to provide breast milk through pumping after 4 months, after which they switch to formula. When asked about reasons for stopping breastfeeding, the top three reasons were: going back to work, lack of support, and difficulties pumping. A socioeconomic study of the mothers revealed that women who choose to breastfeed for longest tend to have a higher level of education and come from more affluent backgrounds. Conversely, those mothers who breastfeed for the least amount of time tend to belong to minority groups and are from poorer backgrounds. It has been proposed that cultural differences also have a significant impact on the duration of breastfeeding.

  Based on the information in the passage, would a mother with a supportive family environment be more or less likely to continue breastfeeding for a full year?

  A. She would be less likely because there are many more influential factors that stop mothers breastfeeding.
  B. She would be more likely because family would be able to help with feeding.
  C. It is impossible to say because the survey only looked at mothers who stopped breastfeeding after 4 months.
  D. She would be more likely because lack of support was one of the top three reasons for mothers stopping breastfeeding.
  D. She would be more likely because lack of support was one of the top three reasons for mothers stopping breastfeeding.

  ---

  Explanation/Reference:

  Lack of support was one of the top three reasons mothers said they stopped breastfeeding. Although the survey only questioned mothers who had stopped breastfeeding after 4 months, the results imply that a more supportive environment would make them more likely to continue breastfeeding.

• Question 753:

  Which of the following glands is known as "master gland"?

  A. Thyroid
  B. Pituitary
  C. Island of Langerhans
  D. Adrenal
  B. Pituitary

• Question 754:

  The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.

  When the battery is being discharged, the following reaction takes place:

  

  Reaction 1

  The electrode reactions, both written as reductions, are shown in Table 1.

  Table 1

  Half-reaction

  E?V)

  PbO2(s) + SO42-(aq) + 4H+(aq) + 2ePbSO4(s) + 2H2O PbSO4(s) + 2e-Pb(s) + SO42-(aq)

  ?.36

  As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.

  Currents as small as 0.1 A can be fatal to humans. If the typical resistance of the human body is 10 k, what is the minimum voltage that could be fatal?

  A. 0.1 V
  B. 1 V
  C. 100 V
  D. 1,000 V
  D. 1,000 V

  ---

  Explanation/Reference:

  Remembering that a k is 103 and using the relationship that V = IR, a current of 0.1 A times a resistance of 10,000 gives a voltage of 1000 V. Choice D is therefore the correct answer. Choice B would have been chosen if you did not convert kiloohms to ohms. Choice A or choice D could have been chosen if you made other conversion errors.

• Question 755:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  The amount of soil on a particular one-acre field down to a depth of one furrow slice weighs 9 ?105 kilograms. Based on Figure 2, how many kilograms of CaCO3 would have to be added to this field to raise the pH from 5 to 6?

  

  A. Option A
  B. Option B
  C. Option C
  D. Option D
  A. Option A

  ---

  Explanation/Reference:

  Beginning at pH 5, the graph tells you how much of a change in pH is produced by adding a given number of milliequivalents of acid or base to 10 grams of soil. You can see by examining the graph that 0.2 milliequivalents of base would raise the pH of the sample from 5 to 6. Since one equivalent equals a thousand milliequivalents, and one kilogram equals a thousand grams, you can convert this ratio from 0.2 milliequivalents per 10 grams, to 0.2 equivalents per 10 kilograms, or

  0.02 equivalents per one kilogram. Since one mole of carbonate can take up two moles of hydrogen ions, a mole of calcium carbonate acts as two equivalents of base. Remember that as the concentration of hydrogen ions goes down, the pH of the solution goes up. The molecular weight of calcium carbonate is 100, so 100 grams of calcium carbonate will provide 2 equivalents of base. We have determined that you need 0.02 equivalents of calcium per kg of soil. Since 2 equivalents are provided by 100 grams of calcium carbonate, 0.02 equivalents are provided by 1 gram. So you need 1 gram of calcium carbonate for each kilogram of soil in the acre furrow slice. This comes to 9 105 grams of calcium carbonate, or 900 kilograms, choice A.

• Question 756:

  Several techniques have been developed to determine the order of a reaction. The rate of a reaction cannot be predicted on the basis of the overall equation, but can be predicted on the basis of the rate-determining step. For instance, the following reaction can be broken down into three steps.

  

  Step 1

  

  (Slow)

  Step 2

  

  (fast)

  Step 3

  

  (fast)

  Reaction 1 In this case, the first step in the reaction pathway is the rate-determining step. Therefore, the overall rate of the reaction must equal the rate of the first step, k1 [A] where k1 is the rate constant for the first step. (Rate constants of the different steps are denoted by kx, where x is the step number.)

  In some cases, it is desirable to measure the rate of a reaction in relation to only one species. In a second-order reaction, for instance, a large excess of one species is included in the reaction vessel. Since a relatively small amount of this large concentration is reacted, we assume that the concentration essentially remains unchanged. Such a reaction is called a pseudo first-order reaction. A new rate constant, k', is established, equal to the product of the rate constant of the original reaction, k, and the concentration of the species in excess. This approach is often used to analyze enzyme activity.

  In some cases, the reaction rate may be dependent on the concentration of a short-lived intermediate. This can happen if the rate-determining step is not the first step. In this case, the concentration of the intermediate must be derived from the equilibrium constant of the preceding step. For redox reactions, the equilibrium can be correlated with the voltage produced by two half-cells by means of the Nernst equation. This equation states that at any given moment:

  

  Equation 1 When

  

  Reaction 2

  Note: R = 8.314 J/K•mol; F = 9.6485 x 104 /mol.)

  What is the effect of increasing the concentration of reactants in a voltaic cell?

  A. The voltage increases, while the spontaneity of the reaction remains the same.
  B. The spontaneity of the reaction increases, but the voltage remains the same.
  C. Both the voltage and the spontaneity of the reaction increase.
  D. The reaction rate increases, but the voltage and spontaneity of the reaction are unchanged.
  C. Both the voltage and the spontaneity of the reaction increase.

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  Explanation/Reference:

  To answer this question, you need to understand the relationship between the concentration of reactants and the spontaneity and voltage of the reaction. First let's review what is meant by voltage. When a reaction takes place in a voltaic cell, the oxidation reaction at the anode produces excess electrons. Meanwhile, reduction at the cathode uses up electrons. For the reaction to continue, the cathode needs an additional supply of electrons. As a result of the two half-reactions, there is a movement of electrons through the wire from the anode to the cathode. The faster the reaction takes place, the greater the electrical force pushing electrons away from the anode and pulling them toward the cathode will be. This pressure on the electrons is called the electromotive force, or EMF. The electromotive force is measured in volts, and so is also known as the voltage. Don't confuse the voltage with the current--the voltage is the pressure on the electrons, while the current is the actual movement of electrons through the wire. The current depends not only on the pressure exerted by the EMF, but also on the amount of resistance of the wire. The voltage or electromotive force created by a redox reaction depends on the number of electrons exchanged in the reaction and the rate at which the reaction takes place. If the rate of the reaction changes, then the voltage or potential also changes. If the concentration of reactants is increased, then the reaction will move more quickly to the right, producing more electrons at the anode and using more at the cathode. This increases the voltage. So an increase in the concentration of reactants will increase the voltage. The spontaneity of a reaction is determined by Gibb's free energy, G. If G is negative, the reaction is spontaneous; if G is positive, the reaction is not spontaneous. There is a direct relationship between the potential of a reaction and its free energy, given by the equation: G = –nFE, where n is the number of moles of electrons transferred in the reaction, F is Faraday's constant and E is the cell voltage. So what will happen to the value of G if we increase the concentration of the reactants? The reaction will proceed more quickly, increasing the flow of electrons, and therefore increasing the voltage, which in turn will make the value of G more negative. As the value of G becomes more negative, the reaction becomes more spontaneous. So choice C is correct; the increased concentration of reactants increases both the voltage and the spontaneity of the reaction.

• Question 757:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below: CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  

  Which of the following is NOT necessarily true about the equilibrium reaction between CO2, C, and CO?

  A. The standard entropy change is positive.
  B. A decrease in pressure at constant temperature would shift the equilibrium to the right.
  C. Addition of CO will shift the equilibrium to the left.
  D. The standard Gibbs' free energy change is negative.
  D. The standard Gibbs' free energy change is negative.

  ---

  Explanation/Reference:

  You should know the relationship G = H ?TS. S for this reaction is positive, because there are more moles of gaseous products than reactants, but there is no information about the sign of H. Since H cannot be determined, neither can G. Choice D is the correct response. Choice A is wrong (that is, the statement is certainly true) because, as was just stated, the entropy change is positive. Choice B is wrong because, according to Le Chatelier's principle, a decrease in pressure will favor the side of the reaction with more moles of gas. In this case, it is the products side, and the equilibrium is said to have shifted to the right. Choice C is wrong because, again, according to Le Chatelier's principle, addition of a product will shift the equilibrium to the left, that is, the reactants side.

• Question 758:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Based on the data in Table 1, which of the four patients most likely has central diabetes insipidus?

  A. Patient A
  B. Patient B
  C. Patient C
  D. Patient D
  C. Patient C

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  Explanation/Reference:

  You're told that a normal value for urine osmolarity is 285 milli-osmoles per liter of H2O. Since ADH increases water reabsorption in the kidneys, patients with diabetes insipidus are expected to have a decreased urine osmolarity. And if normal is 285 milli-osmoles per liter of water, then based on the information in Table 1, you should have concluded that Patient A does not have diabetes insipidus. Patients B, C, and D, have a very low urine osmolarity prior to therapy, indicating that there is something wrong. To answer this question, you must have a good understanding of the mechanisms behind both central and nephrogenic diabetes insipidus. According to the passage, central diabetes insipidus is when ADH itself is either deficient in quantity or quality. Therefore, exogenous supplementation of ADH SHOULD alleviate the symptoms; that is, the kidneys should be able to concentrate urine, and therefore, urine osmolarity should greatly increase only AFTER ADH is administered. It should NOT increase after the 24- hour restriction of fluid intake because water is not being reabsorbed. If you look at the results of the four patients, you'll see only Patient C's urine osmolarity increased after ADH was administered. So, choice C is the right answer. In the case of nephrogenic diabetes insipidus it is the tubules themselves that are defective. Exogenous ADH would be ineffective as therapy since the patient's own ADH is sufficient in quality and quantity.

• Question 759:

  Before birth, the rodent brain is sexually undifferentiated. It is only in the first few days following birth, during a period referred to as the critical period, that the rodent brain differentiates along male or female lines. The hormone testosterone plays a critical role in this development. Specifically, sexual differentiation is determined by the presence of estradiol, an estrogen derivative of testosterone, in certain areas of the brain. Testosterone is converted to estradiol in critical brain cells that contain the enzyme aromatase. To study the effects of testosterone on the neonatal rodent brain, the following experiments were conducted: The above research, combined with additional studies, concluded that testosterone has two "organizational" effects on the male rodent brain: Defeminization Moderate levels of testosterone-derived estradiol during the critical period are sufficient for defeminization of the brain. Defeminization of the rodent brain results in loss of estrogen positive feedback on LH and FSH secretion and the ensuing loss of cyclicity, as well as loss of female sex behavior. Masculinization High levels of estradiol due to high levels of testosterone during the critical period results in masculinization of the brain. Masculinization leads to the induction of male sex behavior including antagonism towards other males and the mounting of females.

  

  In Experiment 2, researchers most likely waited 2-3 months before implanting ovaries:

  A. to allow the rat to recover from previous surgery.
  B. to allow testosterone levels to rise to necessary concentrations.
  C. to wait for the critical period to pass.
  D. to promote the defeminization of the rat brain.
  C. to wait for the critical period to pass.

  ---

  Explanation/Reference:

  The experiments were designed to demonstrate the effects of testosterone on sexual differentiation. The ovaries were transplanted to allow the rodent to demonstrate whether or not it had the ability to cycle normally. Thus, it is important that

  transplantation of the ovaries did not influence the development of the rodent. To make sure that the transplantation of ovaries had no effect on the sexual differentiation of the rodent brains, the ovaries had to have been transplanted once the

  sexual differentiation of the rat brain had already occurred. According to the passage, this differentiation occurs during the critical period which begins a few days after birth. The researchers castrated the neonate and waited for the rodent to

  mature past the critical period before implanting the ovaries.

  Choice A is incorrect because it would not take 3 months for the rat to recover from surgery. Also, the rat is castrated at birth indicating that surgery is not an obstacle. Implanting the ovaries would require another surgery.

  Choice B is incorrect because testosterone levels would not rise in the castrated rat (no testes). Choice D is incorrect because defeminization would be promoted by estradiol (or testosterone converted by aromatase).

• Question 760:

  A student conducts a chemical analysis of the components of a popular soft drink. The beverage label shows that the drink contains carbonated water, phosphoric acid, caffeine, and caramel color, but does not indicate the concentrations of these chemicals.

  

  Dissolved carbon dioxide will react reversibly with water to form carbonic acid. In an attempt to analyze the beverage composition, the student conducts the following experiments on a one liter sample of the beverage.

  Experiment 1

  The sample is placed in a sealed beaker cooled to 10?C and a vacuum is created in the space above the beverage. The gas pumped from this space is passed through a solution of BaCl2, producing a white precipitate. The process

  continues until no more precipitate forms. The precipitate is dried and found to have a mass of 9.5 grams.

  Experiment 2

  The remaining solution left in the sealed beaker is then titrated with 0.01 M NaOH to give the titration curve shown in Figure 1.

  

  Figure 1

  In the body, rapid breathing (hyperventilation) leads to a decrease in the concentration of carbon dioxide in the blood. This will tend to:

  

  A. Option A
  B. Option B
  C. Option C
  D. Option D
  B. Option B

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  Explanation/Reference:

  Removing CO2 would shift the equilibrium shown below towards the left, decreasing the H+ concentration in the blood and thus, increasing the pH. The body uses this technique (hyperventilation) as a means of regulating plasma pH.