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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 globular protein folds in water. Which amino acid side chain is most likely to end up buried in the hydrophobic core?

   • A. Aspartate
   • B. Lysine
   • C. Valine
   • D. Serine
   Ver respuesta y explicación

   Respuesta: C. Valine

   Folding in water pushes nonpolar side chains inward and leaves charged and polar ones on the surface where they can interact with water. Valine has a branched hydrocarbon side chain with nothing to hydrogen bond, so the core is where it costs the least energy. Aspartate and lysine carry charges at physiological pH and serine has a hydroxyl group, so all three prefer the surface.

2. 2. A competitive inhibitor is added to an enzyme assay. What happens to the apparent Km and the Vmax?

   • A. Km increases and Vmax is unchanged
   • B. Km decreases and Vmax is unchanged
   • C. Km is unchanged and Vmax decreases
   • D. Km increases and Vmax decreases
   Ver respuesta y explicación

   Respuesta: A. Km increases and Vmax is unchanged

   A competitive inhibitor binds the active site, so it fights substrate for the same spot. That means you need more substrate to reach half maximal velocity, which shows up as a higher apparent Km. Enough substrate still outcompetes the inhibitor completely, so Vmax is untouched. Pure noncompetitive inhibition is the mirror image: Vmax falls and Km stays put.

3. 3. Complete the sentence.

   In oxidative phosphorylation, the electron transport chain uses the energy released by electron transfer to pump protons into the _____. ATP synthase then makes ATP as those protons flow back down their gradient into the _____.

   Opciones para el hueco 1: mitochondrial matrix, intermembrane space, cytosol

   Opciones para el hueco 2: mitochondrial matrix, intermembrane space, cytosol

   Ver respuesta y explicación

   Respuesta: intermembrane space, mitochondrial matrix

   The electron transport chain is a series of redox reactions, and the energy it releases is stored as a proton gradient across the inner mitochondrial membrane rather than as ATP directly. ATP synthase is the only part that turns that gradient back into chemical energy. This is why an uncoupler that lets protons leak across the membrane keeps electron transport and oxygen consumption running while ATP output collapses.

4. 4. A buffer is made from a weak acid and its conjugate base. Its buffering capacity is greatest when:

   • A. The solution pH is two units below the pKa
   • B. The solution pH is two units above the pKa
   • C. The solution pH equals the pKa
   • D. The weak acid is present in large excess over the conjugate base
   Ver respuesta y explicación

   Respuesta: C. The solution pH equals the pKa

   The Henderson-Hasselbalch equation says pH equals pKa plus the log of the base to acid ratio, so pH equals pKa exactly when that ratio is one. With equal amounts of both forms the buffer has the most room to absorb either added acid or added base before one form runs out. Practically, a buffer is useful roughly one pH unit on either side of its pKa, which is why bicarbonate and phosphate cover different ranges in the body.

5. 5. Which statements about eukaryotic gene expression are correct?

   Marca todas las que correspondan.

   • A. Introns are spliced out of the primary transcript before the mature mRNA leaves the nucleus
   • B. The genetic code is degenerate, so several codons can specify the same amino acid
   • C. RNA polymerase requires a primer to begin synthesis
   • D. A missense mutation changes a codon so a different amino acid is inserted
   • E. Ribosomes read the mRNA from the 3 prime end toward the 5 prime end
   Ver respuesta y explicación

   Respuesta: A. Introns are spliced out of the primary transcript before the mature mRNA leaves the nucleus, B. The genetic code is degenerate, so several codons can specify the same amino acid, D. A missense mutation changes a codon so a different amino acid is inserted

   Splicing, capping and polyadenylation all happen in the nucleus, and only the processed mRNA is exported. Degeneracy is why the third base of a codon often tolerates a change with no effect on the protein, which is the definition of a silent mutation, while a missense mutation swaps the amino acid and a nonsense mutation creates a stop codon. RNA polymerase can start a chain from scratch, unlike DNA polymerase, and ribosomes read mRNA 5 prime to 3 prime.

6. 6. In the proximal tubule of the nephron, glucose crosses the apical membrane into the cell against its own concentration gradient by moving together with sodium. This is best described as:

   • A. Simple diffusion
   • B. Facilitated diffusion
   • C. Primary active transport
   • D. Secondary active transport
   Ver respuesta y explicación

   Respuesta: D. Secondary active transport

   Secondary active transport means the carrier itself burns no ATP. It rides an ion gradient that some other pump paid for. Here the sodium potassium ATPase on the basolateral side spends ATP to keep intracellular sodium low, and the sodium glucose cotransporter uses that inward sodium gradient to drag glucose uphill. Block the ATPase and the sodium gradient collapses, and glucose reabsorption stops even though the cotransporter is untouched.

7. 7. Complete the sentence.

   In an infrared spectrum, a broad absorption centered near 3300 reciprocal centimeters points to an _____ bond, while a strong sharp band near 1700 reciprocal centimeters points to a _____.

   Opciones para el hueco 1: O to H, C to H, C to C double

   Opciones para el hueco 2: carbon carbon double bond, carbonyl group, nitrile group

   Ver respuesta y explicación

   Respuesta: O to H, carbonyl group

   Infrared spectroscopy reports bond vibrations, and the two most useful signals for a first pass are the broad hydroxyl stretch and the sharp carbonyl stretch. The hydroxyl band is broad because hydrogen bonding gives the bond a range of strengths across the sample. Seeing both a broad hydroxyl band and a carbonyl band in one spectrum is the classic hint that you are looking at a carboxylic acid.

8. 8. Which carboxylic acid derivative is most reactive toward nucleophilic acyl substitution?

   • A. An acyl chloride
   • B. An ester
   • C. An amide
   • D. A carboxylate ion
   Ver respuesta y explicación

   Respuesta: A. An acyl chloride

   Reactivity in this family tracks two things: how good the leaving group is and how much the attached atom donates electron density into the carbonyl. Chloride is a weak base and a good leaving group, and it donates very little, so the acyl chloride sits at the top. Nitrogen donates strongly, which makes amides the most stable of the neutral derivatives, and a carboxylate is already deprotonated and negatively charged, so a nucleophile has little reason to attack it.

9. 9. A reaction has a negative delta G under the given conditions. Which statements are correct?

   Marca todas las que correspondan.

   • A. The reaction must proceed quickly
   • B. The reaction is spontaneous as written under those conditions
   • C. The reaction must be exothermic
   • D. An enzyme could speed it up without changing the value of delta G
   • E. Under standard conditions, a negative standard delta G means the equilibrium constant is greater than one
   Ver respuesta y explicación

   Respuesta: B. The reaction is spontaneous as written under those conditions, D. An enzyme could speed it up without changing the value of delta G, E. Under standard conditions, a negative standard delta G means the equilibrium constant is greater than one

   Delta G is about thermodynamics, not speed. A spontaneous reaction can take years if the activation energy is high, and a catalyst lowers that barrier without touching the free energy difference between reactants and products. Spontaneity also does not require heat release, since a positive entropy change at high enough temperature can make delta G negative for an endothermic reaction. Standard delta G is related to the equilibrium constant by delta G naught equals minus RT ln K, so a negative value puts K above one.

10. 10. Circulating thyroid hormone rises because the thyroid gland itself is overproducing. In an intact feedback loop, what happens to TSH released by the anterior pituitary?

    • A. It rises, because hormone levels and TSH move together
    • B. It falls, because the high hormone level suppresses the pituitary and the hypothalamus
    • C. It stays the same, because TSH is regulated only by the hypothalamus
    • D. It rises first and then stays permanently elevated
    Ver respuesta y explicación

    Respuesta: B. It falls, because the high hormone level suppresses the pituitary and the hypothalamus

    The hypothalamus releases TRH, the anterior pituitary releases TSH, and the thyroid releases its hormones, which then feed back to shut down the two levels above. When the gland at the end of the chain is the source of the excess, the feedback signal is strong and TSH goes down. That inverse pattern is the point of a negative feedback loop, and the same logic runs the hypothalamus, pituitary and adrenal axis with cortisol.

11. 11. A vessel behaves as a rigid tube with steady laminar flow. If its radius drops to half while the pressure difference across it stays the same, the flow rate becomes about:

    • A. One half of the original
    • B. One quarter of the original
    • C. One eighth of the original
    • D. One sixteenth of the original
    Ver respuesta y explicación

    Respuesta: D. One sixteenth of the original

    Poiseuille's law says flow rate is proportional to the fourth power of the radius for a given pressure difference, viscosity and length. Halving the radius therefore divides flow by two to the fourth, which is sixteen. That fourth power dependence is why radius changes dominate resistance in the circulation, and why small changes in vessel diameter matter far more than changes in length or viscosity.

12. 12. A student has a headache, takes a painkiller, and the headache goes away. She now reaches for the painkiller sooner whenever a headache starts. In operant terms this is:

    • A. Positive reinforcement
    • B. Negative reinforcement
    • C. Positive punishment
    • D. Classical conditioning
    Ver respuesta y explicación

    Respuesta: B. Negative reinforcement

    In operant conditioning, reinforcement means the behavior becomes more frequent and punishment means it becomes less frequent. Positive and negative describe whether something was added or taken away, not whether it felt good. Here an unpleasant stimulus was removed and the behavior increased, which is negative reinforcement by definition. Classical conditioning would involve pairing two stimuli to produce an involuntary response, not strengthening a voluntary action by its consequence.

13. 13. The smallest change in a stimulus that a person can detect half of the time is called the:

    • A. Absolute threshold
    • B. Signal detection criterion
    • C. Difference threshold
    • D. Point of sensory adaptation
    Ver respuesta y explicación

    Respuesta: C. Difference threshold

    The difference threshold, also called the just noticeable difference, is about telling two stimuli apart, while the absolute threshold is the smallest stimulus that can be detected at all half the time. Weber's law adds that the difference threshold is a constant proportion of the original stimulus rather than a fixed amount, so a heavier weight needs a bigger change before you notice it. The signal detection criterion is a separate idea about how willing someone is to say yes when they are unsure.

14. 14. Researchers record the diets of 2,000 healthy adults, then follow the same people for 20 years and record who develops heart disease. Which statements about this study are correct?

    Marca todas las que correspondan.

    • A. It is a prospective cohort study
    • B. Participants were randomly assigned to a diet
    • C. Participants dropping out over the 20 years can bias the result
    • D. It can measure how many new cases of heart disease appear in each diet group
    • E. It shows that diet causes heart disease
    Ver respuesta y explicación

    Respuesta: A. It is a prospective cohort study, C. Participants dropping out over the 20 years can bias the result, D. It can measure how many new cases of heart disease appear in each diet group

    The exposure was measured before the outcome appeared and the group was followed forward, which makes this a prospective cohort study. That design can measure incidence, meaning new cases over time, which case control studies cannot do directly. What it cannot do is establish causation, because nobody assigned the exposure and people who eat differently may differ in many other ways. Attrition matters because the people who leave a long study are often not a random sample of those who stay.

15. 15. Complete the sentence.

    Blaming a stranger's rudeness on their personality while ignoring what is going on around them is the _____. Explaining your own good grade by your ability but your own bad grade by an unfair test is the _____.

    Opciones para el hueco 1: fundamental attribution error, self-serving bias, just world hypothesis

    Opciones para el hueco 2: actor observer bias, self-serving bias, halo effect

    Ver respuesta y explicación

    Respuesta: fundamental attribution error, self-serving bias

    Attribution questions turn on two axes: whether the cause is placed inside the person or in the situation, and whether the person being judged is you or somebody else. The fundamental attribution error is over-weighting personality when explaining other people. The self-serving bias protects your own self-image by taking internal credit for success and external blame for failure. Both are on the MCAT because they shape how patients and clinicians read each other.

Qué preguntan de verdad las tres secciones de ciencias

El MCAT tiene cuatro secciones. Tres son de ciencias: Chemical and Physical Foundations of Biological Systems, Biological and Biochemical Foundations of Living Systems, y Psychological, Social, and Biological Foundations of Behavior. La cuarta es Critical Analysis and Reasoning Skills, que es lectura pura y tiene su propia página. Cada sección de ciencias tiene ahora mismo 59 preguntas en 95 minutos, y unas 15 de esas 59 son preguntas sueltas que van por su cuenta, sin texto. Las puntuaciones de sección van de 118 a 132, y las cuatro secciones suman un total de entre 472 y 528, con 500 en el medio de la escala. La AAMC puede cambiar el formato, así que confirma los números vigentes en aamc.org antes de montar un calendario sobre ellos.

El contenido sale de las categorías de contenido que publica la AAMC. Lo que más sorprende a la gente es cuánta bioquímica aparece, y que aparece en dos secciones distintas. Los aminoácidos, la cinética enzimática y el metabolismo pueden salir en la sección de biología y otra vez en la de química y física. La física es estrecha y aplicada, sobre todo fluidos y presión, termodinámica, óptica y circuitos, y casi siempre se plantea dentro de un sistema vivo en vez de con un bloque bajando por una rampa.

La sección de psicología y sociología es una sección de contenido de verdad, con su propio vocabulario, no una sección de sentido común, y eso pilla a mucha gente. El condicionamiento, los sistemas de memoria, los umbrales perceptivos, la teoría de la atribución y el diseño de investigación tienen definiciones precisas, y las preguntas dependen de esas definiciones. En las tres secciones, la mayoría de preguntas no son de simple memoria. Te dan un escenario, una gráfica o un conjunto de resultados y te preguntan qué se deduce de ahí, así que saberte un término es solo la mitad de lo que necesitas.

Cómo usar esta muestra

Todas las preguntas de abajo son preguntas sueltas a propósito. Las sueltas son la prueba más limpia de si de verdad te sabes el contenido, porque no hay un texto al lado dándote contexto sin que te des cuenta. Si puedes responder preguntas sueltas en frío, los textos se vuelven mucho más fáciles. Si no, practicar con textos te esconderá la laguna en vez de arreglarla. Los dos tests hermanos enlazados abajo cubren el estilo mixto con textos y la sección de lectura, así que haz los tres y compara dónde pierdes puntos.

Hazla de una sentada y sin apuntes, con unos 90 segundos por pregunta, después lee la explicación de todas, también de las que acertaste. Acertar por suerte y saber no son lo mismo, y solo una de las dos cosas sobrevive al día del examen. Trata cada fallo como un tema, no como un dato. Si fallaste la pregunta de tampones y la de termodinámica, repasa ácido base y equilibrio juntos, porque son el mismo razonamiento aplicado a dos magnitudes distintas.

Una nota sobre qué son estas preguntas. El equipo de StudyPDF escribió las 15 a partir de las categorías de contenido que publica la AAMC. No son ítems de la AAMC, no están copiadas de ningún banco de preguntas ni libro de preparación, y StudyPDF no está afiliada a la AAMC. Son práctica de examen, no orientación médica, y nada de esta página debería usarse para tomar una decisión sobre un paciente real.

Cómo StudyPDF crea tests completos con tu material

Quince preguntas te enseñan qué categoría de contenido tienes floja. No pueden sostener todo un plan de preparación del MCAT, y un banco de preguntas genérico rara vez coincide con lo que subrayan los libros de preparación y los apuntes de clase con los que estudias de verdad.

StudyPDF trabaja con tu propio material. Subes el capítulo de tu libro de preparación, tus diapositivas de bioquímica o el resumen de rutas que hiciste tú, y Bo, el agente de estudio, escribe 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 la cinética enzimática o el diseño de investigación, y seguir qué conceptos sigues fallando.

No necesitas un archivo para empezar. Si no tienes nada que subir, di solo un tema, por ejemplo el ciclo del ácido cítrico o el condicionamiento operante, y Bo escribe un set de práctica con eso. Empezar es gratis.