AAMC CBT9 and 9R OFFICIAL Q&A

[Vihsadas]

This is the official Q&A thread for AAMC CBT9 and 9R.

Please post ONLY questions pertaining to AAMC CBT9 and 9R.
Out of respect for people who may not have completed the other exams, do not post questions or material from any other AAMC exam.

Please see this thread for the rules of order before you post.

Good luck on your MCAT!

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[destroyMCAT]

I don't remember this from class or in my MCAT books.. what is peak area in regards to gas chromatography?

PS # 121. How do you know that is a carbohydrates besides that it has C,H,O. Is that enough to assume that dihydroxyacetone is a carb?

 

[Bumbl3b33]

I don't remember this from class or in my MCAT books.. what is peak area in regards to gas chromatography?

PS # 121. How do you know that is a carbohydrates besides that it has C,H,O. Is that enough to assume that dihydroxyacetone is a carb?

it had the standard CnH2nO formula if i remember correctly

 

[meghaneuron]

item 96:

Why is answer A not the correct answer? I read the AAMC explanation, but I don't quite get it. If they found an agnathan skull with vascular connection, wouldn't that mean that the existing agnathan lost the connection. so, therefore present day agnathans are in degenerate state. How does this question differ from item 95?

I got that question wrong as well. :/ I think AAMC is arguing that ANCIENT agnathans would not possess degenerate axes, PRESENT agnathans would possess degenerate axes, since they once had them (as said in question) but have now lost them. ANCIENT agnathans possess the connection, therefore they have not lost it yet and could not be considered "degenerate." Further, in relation to question #95, notice that they are asking about PRESENT agnathans, not ANCIENT agnathans.

I am struggling to understand this passage myself :/ Thus far, that is where I have gotten. Haha. Hope this helps!

 

[orangeblossom]

could someone PLEASE PLEASE explain #113: Why are high concentrations of sodium included in the dialysate?
Answer: To maintain isotonicity of the dialysate solution with blood.

Data from table 1 (for Sodium)
Normal plasma: 140 mEq/L
Renal failure: 138 mEq/L
Dialysate range, Low: 128 mEq/L, and High: 140 mEq/L

 

[illinichief89]

could someone PLEASE PLEASE explain #113: Why are high concentrations of sodium included in the dialysate?
Answer: To maintain isotonicity of the dialysate solution with blood.

Data from table 1 (for Sodium)
Normal plasma: 140 mEq/L
Renal failure: 138 mEq/L
Dialysate range, Low: 128 mEq/L, and High: 140 mEq/L

You don't want to lose sodium from the blood, so if you have an isotonic solution of sodium in the diasylate then there will be no net movement of sodium across the semipermeable membrane in a normal situation. Actually, in renal failure, it would help because the blood would have an influx of sodium from the diasylate due to the sodium moving down its concentration gradient. This will allow stable maintenance of blood volume.

 

[YouNeverKnow22]

can anyone answer #13 for me? I reasoned that since EtOH had the highest octane rating, it would require the least amount of oxygen...is this correct thinking?

 

[3838]

After taking the MCAT, did you think the PS section of this CBT was easier or harder than the exam you took? the PS section just seemed way too easy...

 

[Synapsis]

BS Passage 2, Q103

I actually got this question (last minute switch), but I'm sure my logic wasn't sufficient. I don't know what a semilogarithmic plot (though I guess I should've been able to figure it out) looks like so I just instead asked, why would the test takers want us to recognize that another plot was needed? This would likely be because the plot in the passage has data points with changes that are hard to interpret at certain values. So a semilogarithmic plot would probably be one that shows the graph in an easier to interpret way. Choice D did this best, so I picked it, switching from C at literally the last minute.

How did others approach this? Any tips for this type of problem in general? It felt like a stretch for me.

 

[Neutral]

BS Passage 2, Q103

I actually got this question (last minute switch), but I'm sure my logic wasn't sufficient. I don't know what a semilogarithmic plot (though I guess I should've been able to figure it out) looks like so I just instead asked, why would the test takers want us to recognize that another plot was needed? This would likely be because the plot in the passage has data points with changes that are hard to interpret at certain values. So a semilogarithmic plot would probably be one that shows the graph in an easier to interpret way. Choice D did this best, so I picked it, switching from C at literally the last minute.

How did others approach this? Any tips for this type of problem in general? It felt like a stretch for me.

I am not quite sure of the distinction between of a semilogarithmic plot and a logarithmic plot, but I treated it as the same as a logarithmic plot and it worked for me.

Essentially for a logarithmic plot you have one of your axes increasing not by some arithmetic constant (0,10,20,30,40 etc), but by a geometric constant, usually 10 (0,1,10,100,1000 etc). The only graph that this held true for was D.

 

[gettheleadout]

#47 in PS is crap.

What does "atomic structure" refer to? Crystalline structure? Like lattice structure? Or the atoms themselves? This is not to mention that C is literally a description of the scattering explanation for why light slows down through transparent materials.

Now, the scattering explanation is incorrect, but knowing that is way beyond the physics we should know. Thus, C should be the correct answer.

For an explanation of the (incorrect) scattering mechanism:

[YOUTUBE]FAivtXJOsiI[/YOUTUBE]

For an explanation of why it's wrong and what might really be happening:

[YOUTUBE]CiHN0ZWE5bk[/YOUTUBE]

Edit: Arguing semantics for "random" in C is idiotic, their explanation shouldn't have even gone there.

 

[Synapsis]

I thought the wording to that question was dumb too. But I figured they meant electrons.

 

[gettheleadout]

My word they really are so incredibly stupid...

Number 7 on PS is straightforward, and I got it correct, because the question stem explicitly says to consider pure substances. But the answer explanation starts talking about hydrogen bonds to water when comparing EtOH vs MTBE. What idiot wrote the answer explanation without reading the freaking question stem? That kind of behavior would get you the question wrong on the test.



Edit: This crap makes me seriously concerned that I'm going to miss points on the real test because answers are keyed incorrectly, and I won't ever know because we don't get to see the key.

 

[gettheleadout]

It's like these explanations are written to confuse students who don't fully understand the material...

#33 in PS explains that bubble compression is adiabatic, but then #34 describes the bubbles as "absorbing heat."

"...work is being done on the bubbles, resulting in the vapor bubble absorbing heat..."

No no no no no.

Unless you understand why that terminology is outright incorrect and misleading, you might be confused about whether the compression is indeed adiabatic. The bubbles aren't absorbing heat, because no heat flow occurs; their internal energy, and thus temperature, rises because energy input is occurring via work.

 

[VitaminVater]

#47 in PS is crap.

What does "atomic structure" refer to? Crystalline structure? Like lattice structure? Or the atoms themselves? This is not to mention that C is literally a description of the scattering explanation for why light slows down through transparent materials.

Now, the scattering explanation is incorrect, but knowing that is way beyond the physics we should know. Thus, C should be the correct answer.

For an explanation of the (incorrect) scattering mechanism:

[YOUTUBE]FAivtXJOsiI[/YOUTUBE]

For an explanation of why it's wrong and what might really be happening:

[YOUTUBE]CiHN0ZWE5bk[/YOUTUBE]

Edit: Arguing semantics for "random" in C is idiotic, their explanation shouldn't have even gone there.

I chose C too, seemed the most logical explanation that an exam testing simple, practical physics knowledge would require.
I definitely didn't even think of picking A because just like the scattering model from the cartoonist's video, the emitted light would go in every direction and like the physicist in the video said the degree of refractive index would vary widely for different wavelengths.

Why does AAMC fail at trying to ask/explain fundamental concepts? They give us one option with the correct jargon but wrong conclusion, and one with incorrect jargon but correct conclusion?

 

[gettheleadout]

I chose C too, seemed the most logical explanation that an exam testing simple, practical physics knowledge would require.
I definitely didn't even think of picking A because just like the scattering model from the cartoonist's video, the emitted light would go in every direction and like the physicist in the video said the degree of refractive index would vary widely for different wavelengths.

Why does AAMC fail at trying to ask/explain fundamental concepts? They give us one option with the correct jargon but wrong conclusion, and one with incorrect jargon but correct conclusion?

That's the thing; both A and C are factually not what's occurring, but that's irrelevant to the problem because it means both A and C could be plausible answers.

However, answer A is worded such that it is ambiguous and unclear, while answer C is not. Therefore, C is correct. The fact that their explanation draws the line at "random" is utterly stupid because 1) there was no definition given of "random" and that concept is complicated, 2) I can argue just as easily that it is "random" by some definition due to quantum interactions between photons and the masses they bounce off of.

It's just a poor question with an incorrect answer.

 

[VitaminVater]

That's the thing; both A and C are factually not what's occurring, but that's irrelevant to the problem because it means both A and C could be plausible answers.

However, answer A is worded such that it is ambiguous and unclear, while answer C is not. Therefore, C is correct. The fact that their explanation draws the line at "random" is utterly stupid because 1) there was no definition given of "random" and that concept is complicated, 2) I can argue just as easily that it is "random" by some definition due to quantum interactions between photons and the masses they bounce off of.

It's just a poor question with an incorrect answer.

Yup, by this point I'm actually kind of glad there's more calculation than concept questions in PS.

 

[gettheleadout]

Yup, by this point I'm actually kind of glad there's more calculation than concept questions in PS.

That's not always the case, just by the way. It varies substantially from test to test.

 

[VitaminVater]

That's not always the case, just by the way. It varies substantially from test to test.

I meant relative to the FLs, from what I gathered it seem the real thing has more calculative questions( or just harder ones )

 

[fizaddou]

Hi all

Question about #40

"In the reaction shown in Eq1 Al(OH)3 acts as what kind of base/acid?"

So I assumed it was Na[AL(OH)3] they were asking about but instead they were asking about Al alone. The reaction in Eq1 is for the formation of the Na[Al(OH)3] complex. I'm confused! What the hell are they asking about?

 

[onedirection]

If oligonucleotides such as mRNA were not degraded rapidly by intracellular agents, which of the
following processes would be most affected?

Why was it
The coordination of cell differentiation during development

 

[pm1]

I don't know how to post figures here.. but I am referring to the graph Filtration rate vs Solute Molecular Weight (daltons)

Kaplan's Explanation says that:

We know from the passage that as solute molecular weight increases, the rate of filtration decreases.

How do we know that? Is it through Table 1?
If it is.. isn't Table 1 only referring to concentrations in plasma and dialysate?

Thank you!

 

[FeinMS]

The problem with Peptide B inhibiting Peptide A's effect on osteoblastin or w/e it's called is a non sense. I was debating between C)remain the same and D) decrease, but ended up choosing C. This question's answer would depend on the assumption that previously synthesized osteoblastin before inhibition would be disappearing (denaturing, destroyed or in whatever means.) I agree that the choice C also depends on the otherwise assumption, but what the bloody hell... If AAMC gave either C or D as a possible choice, it would've been much simpler -_-

 

[FeinMS]

I don't know how to post figures here.. but I am referring to the graph Filtration rate vs Solute Molecular Weight (daltons)

Kaplan's Explanation says that:



How do we know that? Is it through Table 1?
If it is.. isn't Table 1 only referring to concentrations in plasma and dialysate?

Thank you!

I don't recall if the passage indicated the rate of filtration will decrease as MW increase. The passage just said that there's a limit of MW that can be filtrated through the membrane. However, it should be quite intuitive that big things can't go through a pore as easily as small things can do. Use whatever analogy that comes to your mind to imagine shoving a big thing into a small hole and shoving a small thing into a small hole. Which is faster and easier? Smaller one.

If you want more scientific explanation, think about effusion. I know effusion may not be the perfectly same situation as this, but big molecules have smaller effusion rates because of their size leading to smaller velocity (which is less relevant) and not fitting through the effusion hole.

 

[summergirl]

I need help with this question!

BS: Item 104 discrete:

When a biological membrane separates two fluid compartments apart with different solute concentrations, an osmotic pressure differential can arise between the compartments because the membrane:
A) dissolves only nonpolar substances
B) allows the passage of solute particles only
C) allows passage of most solvent particles but not solute particles
D) has electrical charges that attract some particles but not others

I chose A but the correct answer is C. I understand that A is incorrect because osmosis is the movement of SOLVENTS, not solutes, although the AAMC's explanation is that nonpolar substance dissolving in the membrane will not affect the concentration of soluble substance on both side of the membrane. However, I don't see how C is correct. Their explanation is that "the inability of most solute molecules to move across the membrane maintains the different solute concentration on both side of the membrane. However, the solute (water) does move across the membrane". First of all, isn't water the SOLVENT and not the solute? Even if I ignore that part, the osmotic pressure depend on the concentration of the substance, so since movement of the solute or the solvent across the membrane can both change the concentration, wouldn't that make C incorrect? Please enlighten me.

 

[gettheleadout]

I need help with this question!

BS: Item 104 discrete:

When a biological membrane separates two fluid compartments apart with different solute concentrations, an osmotic pressure differential can arise between the compartments because the membrane:
A) dissolves only nonpolar substances
B) allows the passage of solute particles only
C) allows passage of most solvent particles but not solute particles
D) has electrical charges that attract some particles but not others

I chose A but the correct answer is C. I understand that A is incorrect because osmosis is the movement of SOLVENTS, not solutes, although the AAMC's explanation is that nonpolar substance dissolving in the membrane will not affect the concentration of soluble substance on both side of the membrane. However, I don't see how C is correct. Their explanation is that "the inability of most solute molecules to move across the membrane maintains the different solute concentration on both side of the membrane. However, the solute (water) does move across the membrane". First of all, isn't water the SOLVENT and not the solute? Even if I ignore that part, the osmotic pressure depend on the concentration of the substance, so since movement of the solute or the solvent across the membrane can both change the concentration, wouldn't that make C incorrect? Please enlighten me.

A's wrong because the membrane isn't a solvent and doesn't "dissolve" anything. C is correct because when the solutes can't cross the membrane and the solvent (water) can, you get net movement of water (osmosis) occurring due to the difference in solute concentration on either side, i.e. the osmotic pressure differential.

 

[enervate]

Is the only difference between a galvanic and electrolytic cell the direction of electron/current flow?

For those having difficulty with the sonoluminescence passage: had I not been curious and reading about the peacock mantis shrimp just the other night, I would've gotten the questions about heat incorrect. From the Wikipedia article:

The snap can also produce sonoluminescence from the collapsing bubble. This will produce a very small amount of light and high temperatures in the range of several thousand kelvins within the collapsing bubble, although both the light and high temperatures are too weak and short-lived to be detected without advanced scientific equipment. The light emission and temperature increase probably have no biological significance but are rather side-effects of the rapid snapping motion.

Curiosity piqued by this comic at theoatmeal.com.

 

[pinklefluff]

Hey guys,

I suddenly became very confused about something. So I understand that in #2 of the PS section, if the speed of the particle increases by a factor of 2, then the electric force on the particle should be the same. I am wondering if the magnetic force would change? Does the capacitor generate a magnetic field? If it does, does that mean the electric field is decreasing as the capacitor discharges? If there is a magnetic force, if the particle's speed increases by x2, then according to F=qvB then the magnetic force on the particle should double?

Sorry for the long questions... I tried looking on the web for answers but I'm just getting more confused.

Thanks!

 

[FistLength]

2 questions that confused me

I will repeat a post from earlier that was not answered because this was my question:

Question on PS 49:

A 7-N force and an 11-N force act on an object at the same time. Which of the following CANNOT be the magnitude of the sum of these forces?

A) 2N
B) 8N
C) 12N
D) 18N

Says the answer is A, but if both of them act at a 60 degree angle, 5.5 N (from the 11N force) and 3.3 N (from the 7 N force) in opposite directions equals 2. Are they not taking angles into account?

Also question #29. I picked the right answer but I think I picked it for the wrong reason. They mention lifting it with a "constant speed". Well, if speed is constant, then wouldn't the force acting on the object =0 since acceleration is 0? So then F*D=0? That would mean that if constant speed in both directions, then work =0 and they would be equal.

I think I am just missing something really clear but if someone could flesh out my understanding I would really appreciate it.

Never mind I resolved both. By drawing vectors you can see the net force can be any of the choices except A for # 49

Number 29 - work is being done in the sense that it is gaining gravitational potential energy.

 

[stuw]

For question 41:
Q: at which electrode is aluminum produced in a galvanic cell and in an electrolytic cell?

a.) at the anode in both cells
b.) at the cathode in both cells
c.) blah
d.) blah

The correct answer is b. I wanted to know if my logic was correct/suitable for this questions: So I believe in this, we're producing aluminum (in its natural state) from a charged state, implying that it's going under reduction. The reduction process will ALWAYS participate in the cathode no matter what right?

 

[workethic712]

For question 41:
Q: at which electrode is aluminum produced in a galvanic cell and in an electrolytic cell?

a.) at the anode in both cells
b.) at the cathode in both cells
c.) blah
d.) blah

The correct answer is b. I wanted to know if my logic was correct/suitable for this questions: So I believe in this, we're producing aluminum (in its natural state) from a charged state, implying that it's going under reduction. The reduction process will ALWAYS participate in the cathode no matter what right?

it is an electrochemical truth that reduction always occurs at the cathode (Red Cat An Ox acronym).

 

[inasensegone]

It's like these explanations are written to confuse students who don't fully understand the material...

#33 in PS explains that bubble compression is adiabatic, but then #34 describes the bubbles as "absorbing heat."

"...work is being done on the bubbles, resulting in the vapor bubble absorbing heat..."

No no no no no.

Unless you understand why that terminology is outright incorrect and misleading, you might be confused about whether the compression is indeed adiabatic. The bubbles aren't absorbing heat, because no heat flow occurs; their internal energy, and thus temperature, rises because energy input is occurring via work.

I had this exact same thought! It's like their answer explanations are designed to allow these questions to be recycled in the future somehow, because they suck at explaining and in this case (33 & 34) they seemingly contradict each other.

AAMC has to have the absolute laziest answer explanations out there.

 

[TheBoneDoctah]

Does anyone understand this?

 

[Rollerbladdin]

View attachment 180438

Does anyone understand this?

Yes - wavelength given in a pipe open at both ends is: Wavelength = 2L/n; and v = (frequency)(wavelength); plug what you know in and you get the answer shown;

 

[TheJourney]

Can someone explain #9 to me?

What are the coefficients for oxygen and carbon dioxide, respectively, if the equation shown below is balanced?

1CH3OCH3 + O2 ----> H2O + CO2

A) 2 and 1
B) 2 and 2
C) 3 and 1
D) 3 and 2

How is the answer D if they asked for the coefficients of Oxygen and CO2. But the explanation gives the coefficients to H2O and CO2? Am I missing something? Was it suppose to say "for water and carbon dioxide?"

 

[TheJourney]

bump.

 

[TheJourney]

Never mind, I see my mistake. I have to read the questions more carefully.

 

[premed1001]

Nevermind

 

[premed1001]

View attachment 180438

Does anyone understand this?

Yes - wavelength given in a pipe open at both ends is: Wavelength = 2L/n; and v = (frequency)(wavelength); plug what you know in and you get the answer shown;

What?

 

[TheJourney]

What?

The wavelength for a pipe open or closed at both ends is lambda = 2L/n. At n =1, lambda (wavelength) = 2L. Then you just plug in as in the equation.
I'm not sure what the keyword is that makes you know that n must be 1.

 

[premed1001]

The wavelength for a pipe open or closed at both ends is lambda = 2L/n. At n =1, lambda (wavelength) = 2L. Then you just plug in as in the equation.
I'm not sure what the keyword is that makes you know that n must be 1.

So v=f*lambda=2L*F
Where does the F=square root (3gh/PiL) come from

Thanks!

 

[TheJourney]

So v=f*lambda=2L*F
Where does the F=square root (3gh/PiL) come from

Thanks!

That was given in the passage I believe.

 

[premed1001]

That was given in the passage I believe.

OK thanks!

 

[SKation]

The actual oscillating surface in Figure 1 would not remain precisely flat; it would have a half-sine-wave shape. Use this fact to determine the wavespeed v of the fundamental mode of oscillation.
A) v = (2gH)1/2
B) v = (3gH)1/2
C) v = (3gH)1/2/π
D) v = 2(3gH)1/2/π
The wave speed v is given by the product of f and the wavelength λ. Because L is one-half of a wavelength (just like a standing wave in a tube of length L at its fundamental vibration), v will be

Thus, D is the best answer.




Sorry another question...what is a half sine way and how do you get the length of 2L

Hey,

Did you find a proper explanation to this im confused on this same question!

Thanks

 

[SKation]

This is half sine wave ----> U

If u tie a rope at one end and shake the other, u make sine waves, a sine wave is from any point to another point where the pattern repeats (2pi radians) and half of that = half sine wave (pi radians). Think of sine/cosine fns from trig

Finding L comes from harmonics (although I didn't think of this during da exam, i did it differently n got lucky), the length of the tank is 1/2 the first harmonic wavelength. Formula is: L=n*lambda/2 (n= counting #s).

The key to the problem was recognizing "fundamental mode of oscillation" refers to fundamental freq n lambda...

Hey

How would you know how to use the L formula cause I was going based off the fact that we would multiply the frquency by pi/2 because the wave is half a sine wave?

Thanks for any help!

 

[SKation]

Hey Guys,

I just did Practice Test 9, and on the biological reasoning I am a bit confused for question 38 regarding the Peptide A and Experiment 2. I dont understand how it states that OC activity can only be stimulated when Peptide A and OB are present however in the table there is no OC activity when they are present together.

Thanks!!

 

[dorian baltar]

BS Passage #3 Item 111
I have seen some people ask about item 111 already.
On my test, 111 is about why there is a high sodium concentration in the dialysate.
I can see why to maintain isotonicity is the correct answer to this.
Could someone explain in more detail why the other choices are incorrect?
For example, why would the sodium not compensate for the other solutes in blood. I thought that perhaps the sodium was helping to maintain isotonicity between the dialysate and blood by keeping osmotic pressure the same for both solutions.
I think I might just not understand how dialysis works.
Sorry for not presenting a very specific question.
I'm just trying to understand why THIS was the only science question I missed on #9. It seems so easy, but the more I think about it, the harder it is for me.
Even looking at it now, I can see how the correct answer makes the most sense, but I can't abolish the other choices.

 

[DeathandTaxes]

Can someone explain to me why the answer is A, when it clearly states in the passage that the secretor gene doesn't affect expression of erythrocyte surface antigens?

 

[DeathandTaxes]

For 209, can someone explain why incorporating the gene in the genome is necessary? for example, why is B) wrong? We only need this to work on target cells that express the gene.

For 210, can someone provide an answer? I don't have the answer key to this one.

 

[DeathandTaxes]

Bump anyone?

 

[IslandStyle808]

View attachment 182788
For 209, can someone explain why incorporating the gene in the genome is necessary? for example, why is B) wrong? We only need this to work on target cells that express the gene.

For 210, can someone provide an answer? I don't have the answer key to this one.

I don't know how much I can help here (since I think these questions are from 9R and I don't have it). For 209, I think the reason why B is wrong is because you want the gene to get into the nucleus and continuously be produced. A virus works the best in this aspect. You can look up retroviruses and adenoviruses used in gene therapies. They are highly effective because they allow for integration into the genome (I forgot if this was true for adenovirsus, but is definitely true for retro). Integration is helpful because for those cells that divide continuously, it allows for the gene to be copied to the next generation of cells. It also protects the gene from being degraded if just put in by itself. The most effective gene therapy is the longest lasting one.

If the gene were microinjected (assuming it can be put into the nucleus) this will not be integrated into the genome, and thus will be degraded more easily. If the cells were dividing, the gene will be less likely passed on.

I am not sure of 210.

I hope this helps