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Las 15 preguntas de un vistazo

¿Prefieres leer antes de jugar? Abajo están todas las preguntas de la muestra. Las respuestas y explicaciones quedan plegadas hasta que las abras.

1. 1. A person meets the same pathogen for the second time and clears it far faster than the first time. Which feature of the immune system best explains the difference?

   • A. Neutrophils now reach the site of infection sooner than they did before
   • B. The complement system was permanently strengthened by the first exposure
   • C. Memory B and T cells left by the first exposure mount a faster and larger specific response
   • D. Physical barriers such as skin and mucus have adapted to that particular pathogen
   Ver respuesta y explicación

   Respuesta: C. Memory B and T cells left by the first exposure mount a faster and larger specific response

   Innate defenses, meaning barriers, phagocytes and complement, respond the same way every time and keep no record of what they met. The adaptive arm does keep a record, because clonal selection during the first exposure leaves behind long lived memory B and T cells specific to that antigen. On re-exposure those cells expand quickly, so antibody appears sooner and climbs higher, which is the secondary response. That is the effect vaccines are built to produce without the first illness.

2. 2. What is the net yield of glycolysis per molecule of glucose in the cytosol?

   • A. 2 ATP, 2 NADH and 2 pyruvate
   • B. 4 ATP, 2 NADH and 2 pyruvate
   • C. 2 ATP, 2 FADH2 and 2 lactate
   • D. 36 ATP, 2 NADH and 2 pyruvate
   Ver respuesta y explicación

   Respuesta: A. 2 ATP, 2 NADH and 2 pyruvate

   Substrate level phosphorylation makes four ATP in the payoff phase, but two are spent priming the sugar in the investment phase, so the net is two. Glycolysis also reduces two NAD+ to NADH and ends at two pyruvate. None of it needs oxygen, which is why a cell can run glycolysis when the electron transport chain is stalled, as long as it can regenerate NAD+ some other way.

3. 3. Which of the following are true of the tertiary structure of a globular protein?

   Marca todas las que correspondan.

   • A. Hydrophobic side chains tend to cluster in the interior, away from water
   • B. It is determined mainly by hydrogen bonding between backbone amide and carbonyl groups
   • C. Disulfide bridges can form between cysteine residues that are far apart in the sequence
   • D. It refers to the association of two or more separate polypeptide chains
   • E. Salt bridges between oppositely charged side chains help hold the fold together
   Ver respuesta y explicación

   Respuesta: A. Hydrophobic side chains tend to cluster in the interior, away from water, C. Disulfide bridges can form between cysteine residues that are far apart in the sequence, E. Salt bridges between oppositely charged side chains help hold the fold together

   Tertiary structure is the three dimensional fold of one chain, and it is driven by side chain interactions: the hydrophobic effect burying nonpolar residues, disulfide bonds between cysteines, salt bridges, and hydrogen bonds between side chains. Backbone hydrogen bonding is what defines secondary structure, the helices and sheets. Two or more chains coming together is quaternary structure.

4. 4. Complete the sentence.

   In oxidative phosphorylation, electrons passed down the chain are finally handed to _____, and the proton gradient across the inner mitochondrial membrane drives _____.

   Opciones para el hueco 1: carbon dioxide, oxygen, NAD+, pyruvate

   Opciones para el hueco 2: ATP synthase, the citric acid cycle, glycolysis

   Ver respuesta y explicación

   Respuesta: oxygen, ATP synthase

   Oxygen is the terminal electron acceptor and combines with protons to form water, which is why the whole chain backs up without it. The energy released as electrons fall is used to pump protons out, storing energy as an electrochemical gradient. ATP synthase then lets protons flow back down that gradient and uses the flow to phosphorylate ADP, a coupling known as chemiosmosis.

5. 5. In E. coli, glucose is plentiful and lactose is absent. What is the lac operon doing?

   • A. The repressor is inactive, so transcription runs at a high rate
   • B. cAMP is high, so CAP binds and strongly activates transcription
   • C. Lactose binds the repressor and pulls it off the operator
   • D. The repressor sits on the operator and transcription is largely off
   Ver respuesta y explicación

   Respuesta: D. The repressor sits on the operator and transcription is largely off

   The lac operon is under negative control by the repressor and positive control by CAP. With no lactose there is no inducer to lift the repressor, so it stays bound to the operator. Plentiful glucose also keeps cAMP low, so CAP does not bind and gives no boost. Both signals point the same way: do not build enzymes for a sugar that is not there.

6. 6. Two aqueous solutions sit either side of a membrane that lets water through but no solute. Solution A is 0.10 molar glucose and solution B is 0.10 molar sodium chloride. What happens?

   • A. Water moves toward the glucose, because glucose is the larger molecule
   • B. Water moves toward the sodium chloride, because it splits into about twice as many particles
   • C. There is no net movement, because the two molarities are equal
   • D. Water moves toward the glucose, because ionic compounds cannot generate osmotic pressure
   Ver respuesta y explicación

   Respuesta: B. Water moves toward the sodium chloride, because it splits into about twice as many particles

   Osmotic pressure is a colligative property, so it counts dissolved particles and ignores what those particles are. Sodium chloride dissociates into two ions per formula unit, which puts 0.10 molar NaCl near 0.20 osmolar while the glucose stays at 0.10. Water moves toward the side that is more concentrated in particle terms, so it moves toward the salt. The same counting explains freezing point depression and boiling point elevation.

7. 7. Two identical 10 ohm resistors are connected in parallel across a battery. What is the resistance of the pair, and how does it compare with one resistor on its own?

   • A. 20 ohms, which is larger than either resistor alone
   • B. 10 ohms, the same as either resistor alone
   • C. 5 ohms, which is smaller than either resistor alone
   • D. 0 ohms, because parallel paths cancel each other
   Ver respuesta y explicación

   Respuesta: C. 5 ohms, which is smaller than either resistor alone

   Parallel resistors add as reciprocals, so one over the total equals one tenth plus one tenth, and the total is 5 ohms. Adding a parallel path always lowers total resistance, because current gains another route to take. Series resistors do the opposite and simply add. The same reasoning is why opening more parallel vascular beds lowers total peripheral resistance.

8. 8. Which statements about a galvanic cell, also called a voltaic cell, are correct?

   Marca todas las que correspondan.

   • A. The standard free energy change of the cell reaction is positive
   • B. Oxidation happens at the anode and reduction happens at the cathode
   • C. An external power supply is needed to drive the reaction forward
   • D. The standard cell potential is positive
   • E. The salt bridge keeps each half cell electrically neutral as the reaction runs
   Ver respuesta y explicación

   Respuesta: B. Oxidation happens at the anode and reduction happens at the cathode, D. The standard cell potential is positive, E. The salt bridge keeps each half cell electrically neutral as the reaction runs

   A galvanic cell runs a redox reaction that is already favorable, so its standard cell potential is positive and its standard free energy change is negative, the two tied together by delta G naught equals minus n F E naught. Oxidation at the anode and reduction at the cathode holds in every electrochemical cell, so that pairing is not what separates the two types. Direction is what separates them, since an electrolytic cell needs an outside supply to push an unfavorable reaction uphill. The salt bridge moves ions between the half cells to cancel the charge buildup that would otherwise halt the reaction within seconds.

9. 9. Complete the sentence.

   A reaction whose enthalpy change is negative and whose entropy change is positive is _____.

   Opciones para el hueco 1: never spontaneous, spontaneous only at high temperature, spontaneous at every temperature, spontaneous only at low temperature

   Ver respuesta y explicación

   Respuesta: spontaneous at every temperature

   Gibbs free energy change equals enthalpy change minus temperature times entropy change. A negative enthalpy term and a positive entropy term both push that value negative, and absolute temperature is always positive, so the sign never flips. Spontaneous says nothing about speed, though. A reaction can be thermodynamically favorable and still sit untouched for years behind a large activation barrier.

10. 10. Students are told a hard test measures ability in an area where their group is stereotyped as weaker. They score lower than an otherwise identical group told the same test is a problem-solving exercise. This result is usually explained as:

    • A. social loafing
    • B. the bystander effect
    • C. deindividuation
    • D. stereotype threat
    Ver respuesta y explicación

    Respuesta: D. stereotype threat

    Stereotype threat is the drop in performance that appears when someone is reminded of a negative stereotype about a group they belong to, because the resulting worry and self-monitoring consume the working memory the task needs. Nothing about ability changed between the two groups here, only the framing of the task, which is why the instructions alone moved the scores. Social loafing, the bystander effect and deindividuation are all effects of being in a group, and none of them turn on how a task is described to an individual.

11. 11. Which of the following are true of classical conditioning?

    Marca todas las que correspondan.

    • A. A previously neutral stimulus comes to produce a response on its own
    • B. The response is voluntary and is shaped by the consequences that follow it
    • C. Presenting the conditioned stimulus repeatedly without the unconditioned stimulus leads to extinction
    • D. After extinction, the conditioned response can reappear later with no new training
    • E. Learning requires the learner to watch a model being rewarded
    Ver respuesta y explicación

    Respuesta: A. A previously neutral stimulus comes to produce a response on its own, C. Presenting the conditioned stimulus repeatedly without the unconditioned stimulus leads to extinction, D. After extinction, the conditioned response can reappear later with no new training

    Classical conditioning pairs a neutral stimulus with one that already produces a reflexive response until the neutral stimulus produces it alone. Break the pairing and the response fades, which is extinction, and it can come back on its own later, which is spontaneous recovery. Behavior shaped by its consequences is operant conditioning, and learning by watching a model is observational learning.

12. 12. Complete the sentence.

    The system that holds a few items for a matter of seconds while you actively work on them is _____. Recalling a fact more easily in the room where you first learned it is an example of _____.

    Opciones para el hueco 1: working memory, sensory memory, procedural memory

    Opciones para el hueco 2: proactive interference, the serial position effect, context dependent retrieval

    Ver respuesta y explicación

    Respuesta: working memory, context dependent retrieval

    Sensory memory holds a raw impression for well under a second, working memory holds a small handful of items while you manipulate them, and long term memory has no known practical ceiling. Retrieval is a separate problem from storage, since a memory can be stored perfectly well and still be hard to reach without the right cue. Context dependent retrieval is one of those cues, where the surroundings present at encoding are reinstated at recall, and state dependent retrieval does the same with internal state.

13. 13. Read the excerpt, then answer the question. The catalogue is the quiet argument of a museum. Curators speak of hanging a room, of letting one canvas answer another, and visitors read the sequence as though it were natural. It is not. Every wall states a claim about what belongs beside what, and about which century explains which. A visitor who walks the rooms in order absorbs a thesis she was never asked to accept, and rarely notices that a thesis was offered at all. Which of the following best states the main idea of the passage?

    • A. Museums ought to display their collections in strict chronological order
    • B. The arrangement of a museum makes an argument that most visitors take as given
    • C. Curators disagree sharply with one another about which paintings matter most
    • D. Visitors would learn more from reading a catalogue than from walking the galleries
    Ver respuesta y explicación

    Respuesta: B. The arrangement of a museum makes an argument that most visitors take as given

    The passage says every wall states a claim and that visitors absorb the resulting thesis without noticing it, which is the one idea that covers the whole excerpt. Main idea questions reward the choice that spans the passage, not a detail lifted from one line. The option about chronological order and the one about catalogues both recommend something the author never recommends, and the option about curators disagreeing introduces a conflict the passage never mentions.

14. 14. Read the excerpt, then answer the question. Economists once treated household labor as a residual, a thing done in the hours left over after real work. The convention was convenient. Unpaid hours resisted measurement, and what resists measurement tends to fall out of the model. When time use surveys finally arrived, they did not so much discover housework as make it countable, and the finding was greeted less as news than as an accounting problem. The hours had always been there. Only the ledger had changed. The author's remark that the surveys did not so much discover housework as make it countable most nearly implies that:

    • A. the work in question was new to the period the surveys covered
    • B. economists had deliberately concealed the existence of unpaid labor
    • C. the surveys changed what could be measured rather than what was happening
    • D. time use surveys were less accurate than the methods that came before them
    Ver respuesta y explicación

    Respuesta: C. the surveys changed what could be measured rather than what was happening

    The last two sentences say the hours had always been there and only the ledger changed, so the shift was in measurement and not in behavior. Reasoning questions want the smallest step past what the text supports. Deliberate concealment is far stronger than anything the author claims, since he calls the old convention convenient rather than dishonest, and the passage never compares the accuracy of one method with another.

15. 15. Read the excerpt, then answer the question. Defenders of the classics rarely argue that old books are useful. They argue instead that usefulness is the wrong test, and that a culture which asks only what a thing is for will end by keeping nothing it cannot spend. The claim has force, and it also has a convenient shape, since any objection can be dismissed as proof of the very narrowness complained of. An argument that cannot lose is not thereby strong. It has only stopped being an argument. Which best describes the author's attitude toward the defense he describes?

    • A. Sympathetic but critical, since he grants the defense force and then objects to its shape
    • B. Full agreement, since he says outright that the claim has force
    • C. Dismissive, since he holds that old books have no practical use
    • D. Neutral, since he only reports what defenders of the classics say
    Ver respuesta y explicación

    Respuesta: A. Sympathetic but critical, since he grants the defense force and then objects to its shape

    The author concedes that the claim has force, which rules out dismissal, and then complains that a defense treating every objection as evidence for itself has stopped being an argument. That is criticism of the reasoning, not of old books, so he never takes the dismissive position either. Attitude questions on this section usually land on a mixed answer whenever a passage contains both a concession and a complaint.

Qué es el MCAT y cómo se puntúa

El MCAT lo escribe y administra la Association of American Medical Colleges. Tiene cuatro secciones puntuables: Chemical and Physical Foundations of Biological Systems, Critical Analysis and Reasoning Skills, Biological and Biochemical Foundations of Living Systems, y Psychological, Social, and Biological Foundations of Behavior. Cada sección se puntúa en una escala de 118 a 132, así que el total va de 472 a 528, y 500 queda en el punto medio de ese rango. No hay nota de aprobado. La AAMC publica los percentiles de cada puntuación, y cada facultad de medicina de Estados Unidos decide por su cuenta qué quiere ver, así que la única referencia útil es el perfil de puntuaciones de las facultades a las que piensas presentarte.

StudyPDF no está afiliada a la AAMC, y nada de esta página es un producto oficial de la AAMC. Cada pregunta y cada explicación de aquí las escribió el equipo de StudyPDF a partir de las categorías de contenido que publica la AAMC, y los extractos de lectura de las tres últimas preguntas también los escribimos nosotros. Aquí no se ha reproducido ningún ítem de la AAMC ni de ningún otro creador de exámenes. Eso corta por los dos lados. Como las preguntas son nuestras, solo pueden aproximarse a las de verdad, así que úsalas para encontrar lagunas y usa material de práctica oficial de la AAMC cuando quieras una lectura fiable de dónde está de verdad tu puntuación.

El formato es largo. Cada una de las tres secciones de ciencias te da 59 preguntas en 95 minutos, y la sección de razonamiento te da 53 preguntas en 90 minutos, lo que deja un tiempo total sentado de unas siete horas y media contando descansos y registro. La mayoría de preguntas cuelgan de un texto, y solo las secciones de ciencias llevan además un grupo menor de preguntas sueltas independientes. No se resta nada por fallar, así que dejar en blanco siempre es peor que adivinar. Las normas del día del examen, la estructura de descansos, las citas y las ayudas con las tasas sí cambian, así que confirma los detalles vigentes en students-residents.aamc.org en vez de fiarte de un número que hayas leído en un foro.

Cómo usar esta muestra

Las 15 preguntas de abajo siguen en pequeño el propio equilibrio del examen. Cinco vienen de los fundamentos biológicos y bioquímicos, cuatro de los fundamentos químicos y físicos, tres de los fundamentos psicológicos y sociales del comportamiento, y tres son preguntas de análisis crítico y razonamiento. Cada pregunta de razonamiento lleva su propio extracto corto, escrito por nosotros en el registro que usa esa sección, para que puedas practicar la destreza sin tener que buscar antes un texto. El examen real solo usa opción múltiple de respuesta única, así que las preguntas de marcar todas las que correspondan y de completar huecos que hay aquí son un recurso de estudio, no un anticipo del formato. Están en la mezcla porque comprometerte con cada parte de una respuesta deja al descubierto lagunas que cuatro opciones ordenadas pueden esconder. Ninguna de estas es una pregunta de examen real, y ninguna es orientación médica. Esto es práctica de examen, nada más.

Hazla de una sentada y sin apuntes, después lee la explicación de todas las preguntas, también de las que acertaste. En el MCAT una suposición con suerte y una respuesta razonada se ven idénticas en el informe de puntuación y completamente distintas en tu cabeza, así que la revisión es donde se aprende. Trata un fallo como un tema, no como un dato. Si se te cayó la pregunta de presión osmótica y la de termodinámica, el problema seguramente sea que memorizas ecuaciones en vez de leer qué representa de verdad cada variable.

Después de esto, reparte tu tiempo de estudio con honestidad. El repaso de contenido cierra lagunas, pero la puntuación suele moverse con exámenes completos cronometrados y con revisarlos despacio. La sección de razonamiento es la excepción, porque premia leer a diario prosa argumentativa densa más que cualquier lista de contenidos. El tiempo es una destreza aparte. Noventa y cinco minutos para 59 preguntas te dejan poco más de un minuto y medio por pregunta, y la mayoría de la gente pierde puntos al final de la sección más que al principio, así que cuando el contenido esté sólido, practica contra el reloj en vez de sin cronómetro. Los dos tests hermanos enlazados abajo profundizan en las secciones de ciencias y en el razonamiento, así que haz los tres y mira cuál duele.

Cómo StudyPDF crea tests completos con tu material

Quince preguntas te enseñan dónde flojeas. Por sí solas no te preparan, y un banco genérico rara vez coincide con lo que subrayan el libro de preparación, la asignatura o los apuntes con los que estás trabajando de verdad.

StudyPDF va por otro camino y trabaja con tu material. Subes tus libros de preparación del MCAT, tus diapositivas de bioquímica o de psicología, o los resúmenes de capítulo que escribiste tú, y Bo, el agente de estudio, crea tests de práctica completos con esas páginas exactas. Cada pregunta sale de tu material y cada explicación cita de dónde viene la respuesta, así que cuando algo parezca raro puedes ir directo a la fuente y comprobarlo. Puedes generar tests nuevos tantas veces como quieras, centrarlos en un solo área como los aminoácidos, los circuitos o los métodos de investigación, y seguir qué conceptos sigues fallando.

No hace falta que subas nada para empezar. Si no tienes ningún archivo a mano, di solo un tema, por ejemplo química ácido base, fisiología muscular o estratificación social, y Bo escribe un test de práctica con eso. Empezar es gratis.