MCAT-øvespørgsmål

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Alle 15 spørgsmål i overblik

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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
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   Svar: 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
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   Svar: 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 _____.

   Muligheder for felt 1: mitochondrial matrix, intermembrane space, cytosol

   Muligheder for felt 2: mitochondrial matrix, intermembrane space, cytosol

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   Svar: 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
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   Svar: 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?

   Vælg alle, der passer.

   • 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
   Vis svar og forklaring

   Svar: 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
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   Svar: 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 _____.

   Muligheder for felt 1: O to H, C to H, C to C double

   Muligheder for felt 2: carbon carbon double bond, carbonyl group, nitrile group

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   Svar: 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
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   Svar: 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?

   Vælg alle, der passer.

   • 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
   Vis svar og forklaring

   Svar: 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
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    Svar: 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
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    Svar: 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
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    Svar: 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
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    Svar: 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?

    Vælg alle, der passer.

    • 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
    Vis svar og forklaring

    Svar: 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 _____.

    Muligheder for felt 1: fundamental attribution error, self-serving bias, just world hypothesis

    Muligheder for felt 2: actor observer bias, self-serving bias, halo effect

    Vis svar og forklaring

    Svar: 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.

Hvad de tre naturvidenskabelige dele faktisk spørger om

MCAT har fire dele. Tre af dem er naturvidenskab: Chemical and Physical Foundations of Biological Systems, Biological and Biochemical Foundations of Living Systems og Psychological, Social, and Biological Foundations of Behavior. Den fjerde er Critical Analysis and Reasoning Skills, som er ren læsning og har sin egen side. Hver naturvidenskabelig del kører lige nu 59 spørgsmål på 95 minutter, og cirka 15 af de 59 er fritstående spørgsmål, der står for sig selv uden tekst. Delscorerne løber fra 118 til 132, og de fire dele lægger sig sammen til en total mellem 472 og 528, med 500 midt på skalaen. AAMC kan ændre formatet, så bekræft de aktuelle tal på aamc.org, før du bygger en plan op om dem.

Selve indholdet kommer fra de indholdskategorier, AAMC offentliggør. Det overraskende for de fleste er, hvor meget biokemi der dukker op, og at det dukker op i to forskellige dele. Aminosyrer, enzymkinetik og stofskifte kan blive testet i den biologiske del og igen i den kemiske og fysiske del. Fysikken er smal og anvendt, mest væsker og tryk, termodynamik, optik og elektriske kredsløb, og den er næsten altid pakket ind i et levende system frem for en klods, der glider ned ad en skråning.

Delen om psykologi og sociologi er en rigtig indholdsdel med sit eget fagsprog, ikke en sund fornuft-del, og det snyder folk. Betingning, hukommelsessystemer, sansetærskler, attributionsteori og forsøgsdesign har alle præcise definitioner, og spørgsmålene hænger på de definitioner. På tværs af alle tre dele er de fleste spørgsmål ikke simpel genkaldelse. De giver dig et scenarie, en graf eller et sæt resultater og spørger, hvad der følger af det, så det at kende et begreb er kun det halve af det, du skal bruge.

Sådan bruger du smagsprøven

Hvert spørgsmål herunder er et fritstående spørgsmål med vilje. Fritstående spørgsmål er den reneste test af, om du faktisk kan stoffet, for der sidder ingen tekst ved siden af og rækker dig sammenhængen i det stille. Kan du svare på fritstående spørgsmål koldt, bliver tekstbaserede spørgsmål meget lettere. Kan du ikke, vil træning på tekster skjule hullet frem for at lukke det. De to søskendequizzer herunder dækker den blandede tekststil og læsedelen, så kør alle tre, og sammenlign hvor du taber point.

Tag den i ét stræk uden noter, med cirka 90 sekunder per spørgsmål, og læs så forklaringen til hvert spørgsmål, også dem du svarede rigtigt på. At gætte rigtigt og at vide er ikke det samme, og kun det ene overlever prøvedagen. Behandl hvert forkert svar som et emne frem for som et faktum. Missede du bufferspørgsmålet og termodynamikspørgsmålet, så repeter syre-base og ligevægt sammen, for det er den samme tankegang anvendt på to forskellige størrelser.

Et ord om, hvad de her spørgsmål er. StudyPDF-teamet har skrevet alle 15 ud fra de indholdskategorier, AAMC offentliggør. De er ikke AAMC-spørgsmål, de er ikke kopieret fra nogen spørgsmålsbank eller repetitionsbog, og StudyPDF er ikke tilknyttet AAMC. De er eksamenstræning, ikke medicinsk vejledning, og intet på denne side bør bruges til at træffe en beslutning om en rigtig patient.

Sådan bygger StudyPDF fulde øveprøver ud fra dit materiale

Femten spørgsmål kan vise dig, hvilken indholdskategori der er blød. De kan ikke bære en hel MCAT-læseplan, og en generisk spørgsmålsbank rammer sjældent det, som de repetitionsbøger og kursusnoter, du faktisk læser fra, lægger vægt på.

StudyPDF arbejder ud fra dit eget materiale i stedet. Du uploader dit kapitel fra repetitionsbogen, dine slides fra biokemi eller den oversigt over stofskiftevejene, du selv har lavet, og Bo, studieagenten, skriver fuldlange øveprøver ud fra præcis de sider. Hvert spørgsmål bygger på dit materiale, og hver forklaring henviser til, hvor svaret kommer fra, så når noget ser forkert ud, kan du gå direkte til kilden og tjekke det. Du kan lave nye prøver så tit du vil, holde dem på ét område som enzymkinetik eller forsøgsdesign, og følge med i hvilke begreber du bliver ved med at ramme forkert.

Du behøver ikke en fil for at komme i gang. Har du ikke noget at uploade, så nævn et emne i stedet, for eksempel citronsyrecyklussen eller operant betingning, så skriver Bo et øvesæt ud fra det. Gratis at komme i gang.