AAMC BS Self-Assessment Official Q&A

[gettheleadout]

This thread shall serve as the site for discussion of questions from the AAMC Self-Assessment in MCAT Biological Sciences.

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[danny89]

Can somebody briefly explain to me why #58 in the orgo self assessment is answer b?
Thanks

 

[Jepstein30]

Can somebody explain to me why #56 in the orgo self assessment is answer b?
Thanks

Post the question and I'm sure you'll get help. Many of us no longer have access to the self-assessments.

 

[danny89]

Post the question and I'm sure you'll get help. Many of us no longer have access to the self-assessments.

An H NMR was performed on a compound with solvent D2O that yielded 8 signals. If that same compound was treated with CDCl3 instead, and if 9 signals were observed, then the original compound contained:
A.) An acidic hydrogen (the answer)
If it had an acidic proton, shouldn't an H be exchange for a D and yield 1 less signal instead of 1 additional signal?

 

[Jepstein30]

An H NMR was performed on a compound with solvent D2O that yielded 8 signals. If that same compound was treated with CDCl3 instead, and if 9 signals were observed, then the original compound contained:
A.) An acidic hydrogen (the answer)
If it had an acidic proton, shouldn't an H be exchange for a D and yield 1 less signal instead of 1 additional signal?

It only exchanges with D2O due to H-bonding. CDCl3 does not do any H-bonding and therefore, no H exchanging. You have the right concept though.

 

[danny89]

It only exchanges with D2O due to H-bonding. CDCl3 does not do any H-bonding and therefore, no H exchanging. You have the right concept though.

Makes sense. Thank you

 

[danny89]

Question #74 for orgo:
What property of a trihalomethyl group makes it a good leaving group?
Answer: electron-withdrawing group of halogens stablizies the carboanion.
This is for a haloform reaction for a methyl ketone.
My reason: In the tetrahedral intermediate, an arrow goes from an electron from the negative oxygen to reform a C=O double bond and an arrow from the C-CX3(indicating the trihalomethyl group is the leaving group). So my question is the following: How is the carboanion formed (on what carbon is the negative sign on?) And what makes a good leaving group is stabilization through induction effect?
Thanks

 

[Jepstein30]

Question #74 for orgo:
What property of a trihalomethyl group makes it a good leaving group?
Answer: electron-withdrawing group of halogens stablizies the carboanion.
This is for a haloform reaction for a methyl ketone.
My reason: In the tetrahedral intermediate, an arrow goes from an electron from the negative oxygen to reform a C=O double bond and an arrow from the C-CX3(indicating the trihalomethyl group is the leaving group). So my question is the following: How is the carboanion formed (on what carbon is the negative sign on?) And what makes a good leaving group is stabilization through induction effect?
Thanks

Follow the arrows, which represent electrons.

So the oxygen with a negative charge has 3 lone pairs. One lone pair reforms the C=O bond. Now, the carbonyl carbon has 5 bonds.. so we need to break the weakest one (easiest to break). How do we choose the weakest bond? Well, one thing to look at is the stability of the conjugate of possible leaving groups.

Any leaving group will have to absorb a negative charge since the electrons from the bond have to go somewhere. So we need to find a leaving group that will be able to best stabilize that negative charge.

The carbon with the negative charge will be the C in CX3. Now because its surrounded by 3 electron-withdrawing halogens.. that negative charge will be dissipated = stabilized.

Let me know if that's not what you were asking.. I'll try again!

 

[danny89]

Follow the arrows, which represent electrons.

So the oxygen with a negative charge has 3 lone pairs. One lone pair reforms the C=O bond. Now, the carbonyl carbon has 5 bonds.. so we need to break the weakest one (easiest to break). How do we choose the weakest bond? Well, one thing to look at is the stability of the conjugate of possible leaving groups.

Any leaving group will have to absorb a negative charge since the electrons from the bond have to go somewhere. So we need to find a leaving group that will be able to best stabilize that negative charge.

The carbon with the negative charge will be the C in CX3. Now because its surrounded by 3 electron-withdrawing halogens.. that negative charge will be dissipated = stabilized.

Let me know if that's not what you were asking.. I'll try again!

Nailed it. Thanks man.

 

[danny89]

.

 

[FeinMS]

Q35. During the initial skin diving session, when her heart and breathing rates were increased, Sarah noticed that she produced more urine than usual. This was most probably a result of:
A) increased blood pressure caused by her excitement or anxiety.
B) reduced blood pressure caused by her excitement or anxiety
C) absorption of water from the ocean
D) inability to cool the skin through evaporative water loss.
Answer is A

I probably missed this question because I didn't know that sympathetic NS will cause more production of urine. I knew that it relaxes bladder, but I didn't know the effect of sympathetic NS on urine production. Can someone clarify this?

 

[twofac3]

Increased blood pressure = increased glomerular filtration rate = increased urine production

 

[Gauss44]

Biology Self Assessment
#26

The correct answer says that mice have a greater urine volume than humans. How is this correct?

 

[Gauss44]

Biology Self Assessment
#31

Based on the way the question is worded, how should one know that they want you to use the total NET atps (2) from glycolysis, rather than the total number of atp produced (4)?

Now I got this question wrong twice just because of the way it's worded.

 

[gettheleadout]

Biology Self Assessment
#31

Based on the way the question is worded, how should one know that they want you to use the total NET atps (2) from glycolysis, rather than the total number of atp produced (4)?

Now I got this question wrong twice just because of the way it's worded.

The question stem refers to "the energy produced from glycolysis." The energy produced from glycolysis is representative of the ability for further work to be performed as a result of glycolysis being completed, and thus refers to the net ATP produced.

 

[Gauss44]

Biology Self Assessment
#36

Why is "reduce excessive blood pressure" wrong? To me, it was NOT obvious that physical activity was involved based on the question stem.

 

[Gauss44]

The question stem refers to "the energy produced from glycolysis." The energy produced from glycolysis is representative of the ability for further work to be performed as a result of glycolysis being completed, and thus refers to the net ATP produced.

Thank you for responding.

FYI ALL - There are MANY questions above that still need an answer. I just don't want my "thank you" to be misinterpreted. Help is still wanted.

 

[echoyjeff222]

I feel like this is a freaking easy one, but I missed one on the OCHEM passage VII. I said "consume ATP" instead of produce ATP because I thought the ATP was STORED (and thus used) as glucose ...

 

[Gauss44]

Biology Self Assessment
#68

Why not D?

(D fooled me twice now!)

 

[Gauss44]

Biology Self Assessment
#117

I don't understand why the correct answer is A. Doesn't ¨kidney failure¨ usually involve DAMAGE to the kidney? If it did, kidney damage might result from kidney stones resulting from the salts mentioned in answer C. Whats your reasoning?

 

[Gauss44]

Biology Self Assessment
#114

I understand WHY the correct answer is right. My question is, if you got this one right AND have been familiar with the role of PTH on Ca2+, what did you notice that led you away from answer D and toward the right answer?

I got this wrong because I focused on the wrong information. Thats why I want to know where you were focusing, if you got this right AND were familiar with the role of PTH on Ca2+.

 

[justadream]

Organic Self Assessment
#8

How would you distinguish those two compounds (that differ in a C-Br bond) via IR? I've never used IR to distinguish anything with bromine.

(probably not allowed to post an image but the only difference is that one compound has a C-Br bond while the other does not)

 

[K1NG]

Organic Self Assessment
#8

How would you distinguish those two compounds (that differ in a C-Br bond) via IR? I've never used IR to distinguish anything with bromine.

(probably not allowed to post an image but the only difference is that one compound has a C-Br bond while the other does not)

It says except IR

 

[justadream]

It says except IR

The answer is that you can distinguish the two molecules by all the listed choices "except specific rotation" which I understand since the two molecules are chiral. But that answer implies that you CAN use IR to distinguish the two molecules. I was wondering how exactly you would do that.

 

[K1NG]

The answer is that you can distinguish the two molecules by all the listed choices "except specific rotation" which I understand since the two molecules are chiral. But that answer implies that you CAN use IR to distinguish the two molecules. I was wondering how exactly you would do that.

Oh sorry I misread the answer choice lol. Well IR basically measures the strength of the bond. The higher the value of 1/λ, the stronger the bond. A C-H bond (sp3-s) will be ~2900, which is a lot stronger than a C-X bond (around ~800-600).

There are some cases where the C-X can be ~1000 from what I remember depending on the hybridization of the carbon, but the point is that C-H bonds are a lot stronger, and the discrepancy will be seen in IR.

 

[justadream]

Oh sorry I misread the answer choice lol. Well IR basically measures the strength of the bond. The higher the value of 1/λ, the stronger the bond. A C-H bond (sp3-s) will be ~2900, which is a lot stronger than a C-X bond (around ~800-600)

Both compounds will have the ~2900 stretch so I guess the answer is that you look for the presence/absence of a C-X bond at around ~800-600 (where everything is super hard to distinguish anyways)? Isn't this so impractical?

I agree though that optical rotation is the best answer.

 

[K1NG]

Both compounds will have the ~2900 stretch so I guess the answer is that you look for the presence/absence of a C-X bond at around ~800-600 (where everything is super hard to distinguish anyways)? Isn't this so impractical?

I agree though that optical rotation is the best answer.

Haha yeah basically. Like I said in another thread, MCAT questions are going to have answer choices that are no, no, maybe, and yes. Just pick the most obvious (yes) and don't dwell on the maybe.

 

[justadream]

Organic
#67
Is the reason that amide ion would deprotonate the methanol (solvent) first because there is simply an overwhelming amount of methanol compared to methyl iodide?

Because I thought given the choice, a nucleophile would react with methyl iodide first (it's a better electrophile - at least I think so?).

 

[justadream]

Biology #6

The passage indicates that the two types of T-Cells are reciprocally regulated (if one goes up, the other goes down). I thought that they worked together (i.e., if T-Helper cells were activated, they would cause upregulation of T-Killer cell activity).

 

[justadream]

Biology #37

Why does myoglobin presence in the urine mean that kidney is damaged? I understand how it can apply for muscle damage.

Is it simply that a functional kidney would never let myoglobin enter the urine?

 

[ayocaptain628]

Biology #37

Why does myoglobin presence in the urine mean that kidney is damaged? I understand how it can apply for muscle damage.

Is it simply that a functional kidney would never let myoglobin enter the urine?

so myoglobin aka protein
nephron (f(x)'l unit of kidney) is site of filtration yeah? but proteins are too big to go through the mesh network of podocytes in the bowman's capsule
.:. presence of myoglobin aka proteins in the urine would indicate that the kidney is damaged b/c things that shouldn't be filtered are actually being filtered and put in the urine

same thing w/ like glucose, since it's big too
if your piss smells sweet, the podocytes might be faulty

 

[justadream]

Biology #119-122 (Something from the passage)

In experiment 2, "M1/+" is considered diploid. That makes sense. But in experiment 3, why are diploids suddenly "M1 +/+ M2"?

To me, it seems like either there are 4 copies of the gene (impossible) or that somehow each copy is both mutated and wild-type (again, how is that possible)?

 

[Mushrooomboy]

#43

I have a hard time linking cross-reacting with the immune response. Did anyone else have an answer to this question?

 

[Oh_Gee]

someone please explain why it is B? it says NaOH will make a salt COO- group but it says the IR has no peak at 1700. unless it's because the carbonyl oxygen in COOH is more around 1720? that would be very specific though considering there's a range or IR measurements for ketones and COOH
http://www.chem.ucla.edu/~webspectra/irtable.html

 

[Oh_Gee]

It only exchanges with D2O due to H-bonding. CDCl3 does not do any H-bonding and therefore, no H exchanging. You have the right concept though.

Makes sense. Thank you

hey could you guys please explain why the answer couldn't be a a nitrogen containing functionality? is it because acidic is just more likely?

 

[Oh_Gee]

Organic
#67
Is the reason that amide ion would deprotonate the methanol (solvent) first because there is simply an overwhelming amount of methanol compared to methyl iodide?

Because I thought given the choice, a nucleophile would react with methyl iodide first (it's a better electrophile - at least I think so?).

did you ever figure this out? i didn't understand the explanation at all in terms of what they say about pka

 

[justadream]

did you ever figure this out? i didn't understand the explanation at all in terms of what they say about pka

Nope but I think my rationale makes some sense (at least I hope so). There's no other way for me to justify it.

 

[Oh_Gee]

Nope but I think my rationale makes some sense (at least I hope so). There's no other way for me to justify it.

can you elaborate on your thought process more? this question really confuses me

 

[justadream]

can you elaborate on your thought process more? this question really confuses me

Let's say substance_X is super reactive with substance_Y but moderately reactive with substance_Z.

If you have an overwhelming amount of substance_Z, substance_X will react with substance_Z first (before it has a chance to find substance_Y).

 

[MadisonG]

There was a passage involving both oxymercuration-demercuration and hydroboration (passage V), but I thought these topics were no longer tested. Is this a case of them giving us enough info in the passage to be able to answers the questions correctly? Or are we actually responsible for addition reactions?

 

[justadream]

There was a passage involving both oxymercuration-demercuration and hydroboration (passage V), but I thought these topics were no longer tested. Is this a case of them giving us enough info in the passage to be able to answers the questions correctly? Or are we actually responsible for addition reactions?

I think they were just too lazy to get rid of those. Those are not topics we are supposed to know.

 

[Oh_Gee]

There was a passage involving both oxymercuration-demercuration and hydroboration (passage V), but I thought these topics were no longer tested. Is this a case of them giving us enough info in the passage to be able to answers the questions correctly? Or are we actually responsible for addition reactions?

somebody recommended me to just study everything because they might include stuff not on their list by explaining the reaction in the passage

 

[pr2med4lf]

Biology #36:
Sarah noted that her skin blood vessels were usually constricted to conserve body heat in the cold environment of the mountains. However, her skin blood vessels would occasionally dilate for short periods of time. What would be the most probable physiological purpose for this periodic vasodilation?

B. Maintain sufficient oxygenation of cells
C. Reduce excessive blood pressure

Why is B correct instead of C?

 

[justadream]

@pr2med4lf

I got this question wrong myself.

I think you need to understand that dilating your blood vessels at the skin is not really a major mechanism that your body uses to relieve blood pressure. The more important purpose is to ensure that your skin cells get enough oxygen. If you keep your blood vessels at the skin constricted, they are receiving less oxygen that normal. I guess this is okay for short periods but eventually, your cells will need more oxygen.

 

[pr2med4lf]

Thanks @justadream for all your help

Biology #44:
According to Hypothesis 2, the normal balance between effector cells and suppressor cells specific for a certain tissue will most likely be disturbed if the tissue is injected with:

A. cells for that tissue obtained from an identical twin
B. cells from another tissue to which the tissue has already been exposed
C. a foreign substance that cross-reacts with cells of that tissue
D. a foreign substance that does not cross-react with cells of that tissue

I got this question correct but I'm understanding the concepts behind this question as well as I'd like. For instance, I've looked up cross-reaction but can't seem to fit its definition in this context.

 

[pr2med4lf]

Biology #45:

Does acceptance of the mechanism of self-tolerance described in Hypothesis 1 rule out acceptance of the mechanism described in Hypothesis 2?

C. No; suppressor T cell formation can only occur after clonal deletion has occurred
D. No; clones of self-reactive lymphocytes not destroyed by clonal deletion may e controlled by suppressor T cells

Why is C correct and not D?

 

[JMMTB]

Biology #26 Just to make sure I understand why it is A not D. The damaged tissue is what is meaning excreted, not the damaged DNA. Because originally took it as the damaged DNA, which, correct me if I'm wrong, wouldn't be excreted but recycled by the body.

Biology #37 So I understand how a damaged kidney would yield myoglobin in the urine, but not damaged muscle. Is it taking the assumption that the damaged muscle is like the smooth muscle of the bladder? It there was damaged muscle elsewhere would we not see myoglobin in the urine?

 

[VDOadjust]

biology number 95

which of the following hormones are/ is directly required for spermatogenesis
1 LH
2 fsh
3 inhibin
4 testosterone

I chose A testorene b/c answer stem said directly required. I reasoned if both fsh and lh are needed for testosterone for some way or another, so there isn't an answer choice with i 2 and 4, so I chose A, which was testosterone only

 

[VDOadjust]

also, question number 97,

A male taking excess testonterone may become infertile b/c reduced spermatogenesis
according to figure 2, this could result directly from
A) increased inhibin
b) reduced inhibin
c) reduced fsh concentration
d) reduced lh concentration

I though the answer was reduced lh concentration, but they gave answer as reduced fsh concentration. any reasons why?

 

[JMMTB]

95- Spermatogenesis occurs at the sertoli cells (aid in giving nutrients to spermatogoonium). FSH directly acts on Sertoli cells. LH acts on Leydig cells to make testosterone which then acts on Sertoli cells. Thus GnRH and LH act indirectly and FSH and Testosterone act directly.

97- Again as Testosterone increases it acts via a negative feedback loop to the hypothalamus and the anterior pituitary. This reduces FSH and LH, but again it is FSH that acts directly on Sertoli cells aiding in spermatogenesis.