Test di esercitazione MCAT

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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
   Mostra risposta e spiegazione

   Risposta: 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
   Mostra risposta e spiegazione

   Risposta: 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?

   Seleziona tutte le risposte corrette.

   • 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
   Mostra risposta e spiegazione

   Risposta: 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 _____.

   Opzioni per lo spazio 1: carbon dioxide, oxygen, NAD+, pyruvate

   Opzioni per lo spazio 2: ATP synthase, the citric acid cycle, glycolysis

   Mostra risposta e spiegazione

   Risposta: 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
   Mostra risposta e spiegazione

   Risposta: 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
   Mostra risposta e spiegazione

   Risposta: 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
   Mostra risposta e spiegazione

   Risposta: 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?

   Seleziona tutte le risposte corrette.

   • 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
   Mostra risposta e spiegazione

   Risposta: 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 _____.

   Opzioni per lo spazio 1: never spontaneous, spontaneous only at high temperature, spontaneous at every temperature, spontaneous only at low temperature

   Mostra risposta e spiegazione

   Risposta: 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
    Mostra risposta e spiegazione

    Risposta: 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?

    Seleziona tutte le risposte corrette.

    • 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
    Mostra risposta e spiegazione

    Risposta: 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 _____.

    Opzioni per lo spazio 1: working memory, sensory memory, procedural memory

    Opzioni per lo spazio 2: proactive interference, the serial position effect, context dependent retrieval

    Mostra risposta e spiegazione

    Risposta: 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
    Mostra risposta e spiegazione

    Risposta: 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
    Mostra risposta e spiegazione

    Risposta: 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
    Mostra risposta e spiegazione

    Risposta: 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.

Cos'è l'MCAT e come viene valutato

L'MCAT è scritto e somministrato dall'Association of American Medical Colleges. Ha quattro sezioni valutate: Chemical and Physical Foundations of Biological Systems, Critical Analysis and Reasoning Skills, Biological and Biochemical Foundations of Living Systems e Psychological, Social, and Biological Foundations of Behavior. Ogni sezione si valuta su una scala da 118 a 132, quindi il totale va da 472 a 528, e 500 sta al punto medio di quella scala. Non esiste un punteggio di superamento. L'AAMC pubblica i ranghi percentili di ogni punteggio, e ogni facoltà di medicina statunitense decide da sé cosa vuole vedere, quindi l'unico riferimento utile è il profilo dei punteggi delle scuole a cui pensi di candidarti.

StudyPDF non è affiliata all'AAMC, e niente in questa pagina è un prodotto ufficiale AAMC. Ogni domanda e ogni spiegazione qui è stata scritta dal team di StudyPDF partendo dalle categorie di contenuto che l'AAMC pubblica, e anche i brani di lettura delle ultime tre domande li abbiamo scritti noi. Nessun quesito dell'AAMC, né di qualsiasi altro ente d'esame, è riprodotto in questa pagina. La cosa vale in entrambi i sensi. Siccome le domande sono nostre, possono solo avvicinarsi all'esame vero, quindi usale per trovare le lacune e usa il materiale di esercitazione ufficiale dell'AAMC quando vuoi una misura affidabile di dove sta davvero il tuo punteggio.

Il formato è lungo. Ognuna delle tre sezioni scientifiche ti dà 59 domande in 95 minuti, e la sezione di ragionamento ti dà 53 domande in 90 minuti, il che porta il tempo totale seduto a circa sette ore e mezza contando pause e registrazione. La maggior parte delle domande è appesa a un brano, con un gruppo più piccolo di domande discrete indipendenti solo nelle sezioni scientifiche. Una risposta sbagliata non toglie punti, quindi lasciare in bianco è sempre peggio che tirare a indovinare. Le regole del giorno d'esame, la struttura delle pause, la prenotazione e le agevolazioni sulle tariffe cambiano, quindi verifica i dettagli aggiornati su students-residents.aamc.org invece di fidarti di un numero letto in un forum.

Come usare questo assaggio

Le 15 domande qui sotto riproducono in piccolo l'equilibrio dell'esame. Cinque vengono dalle basi biologiche e biochimiche, quattro dalle basi chimiche e fisiche, tre dalle basi psicologiche e sociali del comportamento, e tre sono domande di analisi critica e ragionamento. Ogni domanda di ragionamento porta con sé un breve brano, scritto da noi nel registro che quella sezione usa, così puoi allenare l'abilità senza dover prima cercare un testo. L'esame vero usa solo domande a scelta multipla con una sola risposta, quindi qui le domande a risposta multipla e il completamento di frasi sono uno strumento di studio e non un'anteprima del formato. Sono nel mix perché impegnarsi su ogni parte di una risposta fa emergere lacune che quattro opzioni ordinate riescono a nascondere. Nessuna di queste è una domanda d'esame reale e nessuna è un'indicazione medica. Questa è esercitazione per l'esame, niente di più.

Fallo tutto d'un fiato senza appunti, poi leggi la spiegazione di ogni domanda, comprese quelle a cui hai risposto giusto. All'MCAT una risposta indovinata per fortuna e una ragionata sono identiche sul referto del punteggio e completamente diverse nella tua testa, quindi è nella revisione che si impara. Considera un errore un argomento e non un dettaglio. Se hai perso la domanda sulla pressione osmotica e quella sulla termodinamica, il problema è probabilmente che stai memorizzando equazioni invece di leggere cosa rappresenta davvero ogni variabile.

Dopo, dividi il tuo tempo di studio con onestà. Il ripasso dei contenuti chiude le lacune, ma il punteggio si muove di solito grazie ai test completi a tempo e alla loro revisione fatta con calma. La sezione di ragionamento è l'eccezione, perché premia la lettura quotidiana e costante di prosa argomentativa densa più di qualsiasi elenco di contenuti. Il tempo è un'abilità a parte. Novantacinque minuti per 59 domande ti lasciano poco più di un minuto e mezzo ciascuna, e la maggior parte delle persone perde punti alla fine di una sezione invece che all'inizio, quindi quando i contenuti sono solidi allenati con il cronometro invece che senza. I due quiz gemelli qui sotto approfondiscono le sezioni scientifiche e il ragionamento, quindi falli tutti e tre e guarda quale fa più male.

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Quindici domande possono mostrarti dove sei fragile. Da sole non bastano a prepararti, e una banca dati generica raramente corrisponde a ciò su cui insistono il manuale di ripasso, il corso o gli appunti delle lezioni che stai davvero seguendo.

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