Medical Tests MCAT-TEST Online Practice Questions and Exam Preparation

Medical Tests MCAT-TEST Online Questions & Answers

• Question 581:

  A deep-sea research module has a volume of 150 m3. If ocean water has an average density of 1,025 kg/m3, what will be the buoyant force on the module when it is completely submerged in the water? (Note: The acceleration due to gravity is 9.8 m/s2.)

  

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

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  Explanation/Reference:

  Archimedes' principle states that the buoyant force exerted by a fluid on a submerged object is equal to the weight of the fluid displaced. In this case, the submerged object is the research module and the fluid is ocean water. Mathematically, the buoyant force is given by F = mwg, where mw is the mass of the water and g is the acceleration due to gravity. Since we are not given the mass of the water displaced, we express it as mw = pwVw, where pw is the density of the water and Vw is the volume of the water displaced. But the volume of the water displaced is identical to the volume of the submerged module, V0. Hence, mw = pwVw, = pwV0, and F = mwg = pwV0g. Since the answer choices are far apart, we round off the values given in the question and calculate F = pwV0g (1,000 kg/m3 )(150 m3)(10 m/S2 ) = 1.5 X 106 N.

• Question 582:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  Ozone molecules in the stratosphere absorb ultraviolet radiation and photodissociate into an oxygen molecule (O2), and a free oxygen atom (O). Which of the following best describes this phenomenon?

  A. The wavelength of the ultraviolet radiation is greater than the cutoff wavelength needed for photodissociation.
  B. The frequency of the ultraviolet radiation is greater than the cutoff frequency needed for photodissociation.
  C. The frequency of the ultraviolet radiation is less than the cutoff frequency needed for photodissociation.
  D. The wavelength of the ultraviolet light is less than the cutoff frequency.
  B. The frequency of the ultraviolet radiation is greater than the cutoff frequency needed for photodissociation.

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  Explanation/Reference:

  This question requires an understanding of the energy of photons. The energy is related to the frequency by the following equation, where h is Planck's constant:

  

  Higher energy photons have a higher frequency, and shorter wavelength. The frequency is inversely related to the wavelength according to the following equation:

  

  Thus, the cutoff energy for photodissociation would be related to a minimum frequency (or a maximum wavelength) of light.

• Question 583:

  When humans are submerged in water, the mammalian dive reflex acts to alter circulation. Heart rate decreases, blood flow to the extremities is reduced, and mean arterial blood pressure is increased. These accomodations lead to:

  A. decreased oxygen demand by the tissues.
  B. increased heat retention by the body.
  C. decreased partial pressure of oxygen in the blood.
  D. increased venous return.
  B. increased heat retention by the body.

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  Explanation/Reference:

  The question stem describes the effects of the mammalian dive reflex, indicating that blood flow to the extremities is reduced. This would increase heat retention of the body. Choice A is incorrect because oxygen demand by the tissues

  depends on the metabolic rate of the tissues, which is not affected by changes in blood pressure, heart rate, or blood flow to the extermities. Choice C is incorrect because the dive reflex will not lead to a decrease in oxygen partial pressure.

  The partial pressure of oxygen will decrease during a dive because the individual is holding their breath, not because of the accomodations of the dive reflex.

  Choice D is incorrect because venous return, the amount of blood returned to the heart by the venous circulation, will not be increased. If stroke volume (the volume of blood pumped with each beat) remains constant, the decrease in heart

  rate caused by the dive reflex would lead to a decrease in venous return.

• Question 584:

  Which titration curve would be produced by titrating 25 mL of a 0.1 N weak base with a 0.1 N strong acid?

  

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

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  Explanation/Reference:

  In this question you are asked about the titration of a weak base with a strong acid. The points marked by the dotted lines in each graph are the equivalence points. These are the points at which the acid has been totally neutralized by the added base. To decide which of these graphs is accurate, there are two things you need to know. First, you need to decide how much of a weak base is required to neutralize a strong acid. Second, you must know what the pH will be after the acid has been completely neutralized. First, let's consider the amount of strong acid needed to neutralize a weak base. The question tells you that both the weak base and the strong acid are 0.1 N. Since the weak base is only partially dissociated, the actual hydroxide ion concentration of the base will be considerably lower than the hydrogen ion concentration of the strong acid. For this reason, it may seem likely that the volume of acid that you would have to add to the base to bring about neutralization would be lower than the volume of the base you started with. In choices A and B, only 15 mL of acid are added to bring 25 mL of base to the equivalence point. However, the problem with this line of reasoning is that as soon as the small amount of free hydroxide in the basic solution has been neutralized, more of the base will dissociate and there will be more hydroxide. As you continue to add acid, eventually all of the base will dissociate. Thus, you'll end up having to add enough acid to neutralize all of the hydroxide in the base, just as if it were a strong base. Since the number of equivalents are equal in this case due to the equal normalities, the neutralization of 25 mL of weak base will require a full 25 mL of strong acid and that means that you can eliminate choices A and B. Next, you'll have to decide what the pH of the neutralized solution will be. When the base has been completely neutralized, a solution of salt water remains. But the cation, since it comes from a weak base, has a strong tendency to recombine with hydroxide ions from the water. The anion, since it comes from a strong acid, remains completely dissociated. This means that the process of hydrolysis takes place; the cation combines with the hydroxide from water molecules while the resulting hydrogen ions remain free in solution. Thus, a neutralized solution formed from a weak base and a strong acid will be slightly acidic and choice D is correct.

• Question 585:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  Would the walls of the atria or ventricles expected to be thicker?

  A. Atria, because blood ejection due to atrial contraction is high.
  B. Atria, because blood ejection due to atrial contraction is low.
  C. Ventricles, because ventricular stroke volume is high.
  D. Ventricles, because ventricular stroke volume is low.
  C. Ventricles, because ventricular stroke volume is high.

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  Explanation/Reference:

  Thicker walls in a particular part of the heart would indicate that it is more muscular, and therefore is more efficient or forceful during its contraction. This immediately rules out answer choices B and D, as they suggest that the thicker walled chamber would be less efficient or forceful. It should be known from the biology review (BIO 7.2) that the ventricles are more muscular (thicker-walled), but in the event that this is not initially known, information regarding the function of both the atria and the ventricles from the passage may help lead to this conclusion. First, systole, the period of contraction, refers to the period of contraction of the ventricles not the atria (paragraph 1, line 5). This would indicate that the contraction of the ventricles might be more relevant in some way. Second, during diastole, atrial contraction occurs after most of the ventricle is already filled (paragraph 3, sentence 4) and serves to push the small amount of blood necessary to complete the filling into the ventricles. Since the atrial contraction does not need to move a large amount of blood (the ventricle is already mostly full), it does not need to be as muscular.

• Question 586:

  The rich analyses of Fernand Braudel and his fellow Annales historians have made significant contributions to historical theory and research. In a departure from traditional historical approaches, the Annales historians, assume (as do Marxists) that history cannot be limited to a simple recounting of conscious human actions, but must be understood in the context of forces and material conditions that underlie human behavior. Braudel was the first Annales historian to gain widespread support of the idea that history should synthesize data from various social sciences, especially economics, in order to provide a broader view of human societies over time (although Febvre and Bloch, founders of the Annales school, had originated this approach). Braudel conceived of history as the dynamic interaction of three temporalities. The first of these, the evenementielle, involved short-lived dramatic "events", such as battles, revolutions and the actions of great men, which had preoccupied traditional historians like Carlyle. Conjonctures was Braudel's term for larger cyclical processes that might last up to half a century. The longue duree, a historical wave of great length, was for Braudel the most fascinating of the three temporalities. Here he focused on those aspects of everyday life that might remain relatively unchanged for centuries. What people ate, what they wore, their means and routes of travel -- for Braudel these things create "structures" which define the limits of potential social change for hundreds of years at a time. Braudel's concept of the longue duree extended the perspective of historical space as well as time. Until the Annales school, historians had taken the juridical political unit the nation-state, duchy, or whatever as their starting point. Yet, when such enormous timespans are considered, geographical features may well have more significance for human populations than national borders. In his doctoral thesis, a seminal work on the Mediterranean during the reign of Philip II, Braudel treated the geohistory of the entire region as a "structure" that had exerted myriad influences on human lifeways since the first settlements on the shores of the Mediterranean Sea. And so the reader is given such arcane information as the list of products that came to Spanish shores from North Africa, the seasonal routes followed by Mediterranean sheep and their shepherds, and the cities where the best ship timber could be bought. Braudel has been faulted for the imprecision of his approach. With his Rabelaisian delight in concrete detail, Braudel vastly extended the realm of relevant phenomena; but this very achievement made it difficult to delimit the boundaries of observation, a task necessary to beginning any social investigation. Further, Braudel and other Annales historians minimize the differences among the social sciences. Nevertheless, the many similarly-designed studies aimed at both professional and popular audiences indicate that Braudel asked significant questions which traditional historians had overlooked.

  Which of the following statements is most in keeping with the principles of Braudel's work as described in the passage?

  A. All written history is the history of social elites.
  B. The most important task of historians is to define the limits of potential social change.
  C. Those who ignore history are doomed to repeat it.
  D. People's historical actions are influenced by many factors that they may be unaware of.
  D. People's historical actions are influenced by many factors that they may be unaware of.

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  Explanation/Reference:

  Choice D is the correct answer to this Application question because the author suggests in the first paragraph that one fundamental principle of Braudel's work is that history must be understood in the context of forces and material conditions that underlying human behavior. So the assertion that historical actions are influenced by forces which individuals may be unaware of is perfectly consistent with Braudel's principles. Choice A, on the other hand, is off-base because neither written history nor social elites are mentioned in the passage. Choice B is incorrect because defining the limits of potential social change in the longue duree was but one aspect of Braudel's work and certainly not the historian's most important task. Finally, Choice C is a clich?and not particularly relevant to nor descriptive of Braudel's analysis.

• Question 587:

  The resistance of a resistor is defined as the ratio of the voltage drop across it to the current passing through it. The resistance of a resistor can be measured using the circuit illustrated in Figure 1.

  

  Figure 1

  In the above circuit, a variable voltage source with negligible internal resistance is connected to a resistor. The voltage across the resistor is measured by a voltmeter and the current through the resistor is measured by an ammeter.

  

  Additional resistors may be added to the circuit. The total resistance can be calculated as follows: If and are two resistances of two resistors, then the total resistance is given by = + when the resistors are connected in

  

  series, and by 1/ = 1/ + 1/ when the resistors are connected in parallel.

  

  Circuits similar to the one above are used in the common household appliance known as the toaster. The rate by which energy in the form of heat is dissipated by the resistor equals , where I is the current that passes through the resistor and R is the resistance of the resistor. Energy is dissipated in a resistor because moving electrons collide with atoms in the resistor, causing the atoms to vibrate.

  In order for the ammeter to have a very small effect on the current flowing through the resistor, the ammeter should:

  A. be connected to the resistor with insulated wire.
  B. be connected as close as possible to the positive terminal of the voltage source.
  C. have a very low resistance.
  D. be sensitive to currents flowing in either direction around the circuit.
  C. have a very low resistance.

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  Explanation/Reference:

  Ammeters, like all components of electric circuits, have some internal resistance. Therefore, adding an ammeter to the circuit is like adding another resistor in series. If the ammeter has a large resistance, then the current flowing through the resistor will be significantly reduced when it is added to the circuit. If it has a small resistance, the current will be only slightly affected; so choice C is correct. Choice A is wrong. Insulated wires are wires surrounded by a nonconducting material, which prevents unintended contact with the current. Insulating the wires won't directly affect the current through the resistor and won't affect the way that the ammeter works. Choice B is wrong as well. The current in the wire to the right of the resistor is the same as the current to the left of the resistor, so an ammeter will function identically at either location. In both cases, the ammeter is in series with the resistor. Choice D is also wrong. An ammeter that can detect current in either direction might be useful. But whether it can or not is unrelated to the ammeter's effect on the current flowing through the resistor.

• Question 588:

  

  The amino acid histidine has a nitrogen-containing side chain. The pKa for the carboxylic acid group, imidazole side chain, and the primary amine has a value of 1.8, 6.0, and 9.0, respectively. Which of the following structures accurately reflects the predominate form of histidine at a normal physiological pH of 7.4?

  

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

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  Explanation/Reference:

  This question requires the examinee to apply the Henderson-Hasselbalch equation to determine the most common structure of the amino acid histidine at a normal physiological pH. pH = pKa + log[A?HA] To answer this question, you must figure out the predominate form of each acidic substituent group. Carboxylic acid group: 7.4 = 1.8 + log[A?HA]. The logarithm of the ratio of the conjugate base to the acid must equal 5.6. That means the concentration of the conjugate base is

  5.6 orders of magnitude greater (105.6) than the concentration of the acid. Thus, the carboxylic acid will be in the conjugate base form. Imidazole side chain: 7.4 = 6.0 + log[A?HA]. The concentration of the conjugate base is 1.4 orders of

  magnitude greater than the concentration of the acid.

  Therefore, the conjugate base will be the predominate form. Primary amine group: 7.4 = 9.0 + log[A?HA]. The log[A?HA] is equal to -1.6, which means the concentration of the protonated form is 1.6 orders of magnitude greater than the

  concentration of the conjugate base. Accordingly, histidine will have its carboxylic acid and imidazole side chain in their conjugate base form while the primary amine will remain as neutral R-NH2. Thus, C is the correct answer.

• Question 589:

  In the small intestine, most amino acids are transported from the lumen of the gut into the epithelial cells against their concentration gradient. This movement of amino acids is coupled to the movement of sodium ions down their concentration gradient in a form of transmembrane movement called:

  A. simple diffusion.
  B. facilitated diffusion.
  C. secondary active transport.
  D. primary active transport.
  C. secondary active transport.

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  Explanation/Reference:

  Secondary active transport is defined as the transport of a material across the cell membrane against its concentration gradient without direct hydrolysis of ATP. The energy required to drive this active transport is acquired by harnessing favorable concentrations gradients, like the flow of sodium ion into a cell. Note that the sodium gradient is created by the primary active transport of sodium out of the cell by the sodium/ potassium pump. Choice A is incorrect because simple diffusion is the movement of materials down a concentration gradient through the membrane without the assistance of pumps or protein channels. Small molecules () and non-polar molecules (steroids) can move into a cell by simple diffusion. Choice B is incorrect because facilitated diffusion is movement of a material along its concentration gradient through protein channels. Ions can move in and out of the cell through specific ion channels. Choice D is incorrect because primary active transport involves transport against a concentration gradient driven directly by ATP hydrolysis. The sodium/potassium pump is an ATPase (hydrolyzes ATP).

• Question 590:

  Myoclonic epilepsy and ragged-red fiber (MERRF) is an extremely rare disorder that affects neuromuscular systems. MERRF results from a mutation in mitochondrial DNA (mtDNA) that impairs protein synthesis, oxygen consumption, and energy production. When an affected male and a normal female reproduce, which of the following best predicts the expected phenotypic ratios of the offspring?

  A. None of the offspring will be affected.
  B. All males and no females will be affected.
  C. Half of males and half of females will be affected.
  D. One-fourth of the offspring will be affected.
  A. None of the offspring will be affected.

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  Explanation/Reference:

  This question is asking the examinee to determine offspring phenotypes for a gene that is located on mitochondrial DNA. Since mtDNA follows maternal inheritance, all the offspring receive their mitochondrial DNA from the mother. In this

  case, the MERRF-causing mutation is located on the father's mtDNA. Therefore, the mutations are not passed on to the offspring. Since all of the offspring will receive functioning mtDNA from the mother, none of the offspring will inherit

  mutated genes causing MERRF.

  Thus, answer choice A correct.