Why the MCAT Bio / Biochem section is so critical and how to read a passage, Part III

Note: this post assumes familiarity with Part I and Part II of this series

Welcome back to our deep-dive into analyzing BB passages on the MCAT like a boss.

3d model of the SUSD2 protein we are dealing with

In Part I, we made a passage map dealing with sushi proteins (not really, but you know what I mean) that brought relationships, results, and processes to life graphically. Recall how we didn’t want to look back at the passage again — it’s an absolute waste of your precious time to spend all that time carefully analyzing the passage to come up with a summary only to go back to the passage anyway. Don’t worry if your restated question stems or Frankenstein sentences that concatenate your restated question stem with each answer aren’t perfect the first time you try this technique. Never give up! We then used this passage map to tackle a prototypical question type in Part II. Today, we will go over a couple more questions related to this passage to see just how powerful these techniques are.

Let’s translate the question into the relationship equation format we’ve been using. Everything after “Although” in the first sentence until the comma after “longevity” (subordinate clause) can be rephrased as:

SUSD2 ↑ ⟼ invasiveness ↑

while the rest of that first sentence (the independent clause) can be distilled to:

SUSD2 ↑ ⟼ certain tumors↓

I oversimplified “reversal of tumorigenic phenotypes” into “certain tumors — decrease [as indicated by the ↓]” because this a trend you’re going to notice over and over again: the MCAT wants you to trip over words and phrases that sound scary but actually are mere synonyms for what you just read. Tumorigenic phenotypes just means physically realized tumors, and that’s the only type of tumor we are familiar with. So now let’s make a restated question stem: SUSD2 sometimes accelerates cancer and other times reverses can be explained by…

And we are ready to construct our Frankenstein questions (see Tip 6 in Part II of this series) by substituting in one answer choice at a time.

“A. SUSD2 sometimes accelerates cancer and other times reverses can be explained by the fact that: SUSD2 isn’t expressed in cancerous cells that demonstrate reversal of tumorigenic phenotypes.”

You know that uncomfortable, gnawing feeling you may have experienced when you read that Frankenstein sentence? Like a little warning that something is off. That’s your sharpening intuition at play. These techniques I’m sharing with make it easy for your brain to quickly get to the right answer by eliminating these wrong ones because you’re not straining your working memory capacity with irrelevant details by laser focusing in the essence (the restated question stem) and if the answer choice satisfies that restatement (the Frankenstein sentence we made). If it’s not obvious why A is wrong, or any of your Frankenstein sentences are, for that matter, then you need to restate the answer choice into a way that makes the whole sentence easier to evaluate as true or false.

“A. SUSD2 sometimes accelerates cancer and other times reverses cancer can be explained by the fact that: SUSD2 ↓ ⟼ certain tumors ↓”

How about now? OK, let’s substitute the mathematical shorthand for the question stem to really drive it home:

“A. SUSD2 ↑ ⟼ invasiveness ↑ and SUSD ↑ ⟼ certain tumors ↓ can be explained by the fact that: SUSD2 ↓ ⟼ certain tumors ↓”

I love this technique so much because once you master it, you’re guaranteed to get the right answer. Let’s continue.

What is B saying, in mathematical shorthand? If TGFß ↑, then SUSD2 ↑ ⟼ invasiveness ↑. First of all, is this answering the question? Does this explain the fact that SUSD2 sometimes accelerates cancer and other times reverses tumorigenic phenotypes? That’s why you have to construct those Frankenstein sentences; it lets you quickly see if the answer choice is even relevant:

“B. SUSD2 sometimes accelerates cancer and other times reverses cancer can be explained by the fact that: if TGFß ↑, then SUSD2 ↑ ⟼ invasiveness ↑”

There are two reasons to eliminate the answer: (1) it doesn’t even answer the question. The perplexing contradictory behavior of SUSD2 cannot be explained by the observation that in the presence of TGFß, SUSD2 leads to cancer. Also (2): our Frankenstein statement for B contradicts our passage map, specifically when we summarized paragraph 2: TGFß ↑ ⟼ SUSD2 ↓. Also, when researchers incubated the Ishikawa cells (which overexpress SUSD2) in TGFß, the cells exhibited more apoptosis.

Let’s move on to our Frankenstein C with a direct substitution of the answer choice.

“C. SUSD2 sometimes accelerates cancer and other times reverses cancer can be explained by the fact that: SUSD2 modulates specific signal transduction processes that are determined by cell type and differentiation state”

The beauty of this concatenation method (see Tip 6 in Part II) is unfathomable. Without even restating the answer choice, we can see that it satisfies the restated question stem when taken as a whole. In other words: SUSD2 triggers processes that depend on what kind of cell it’s in and how differentiated it is, explaining why SUSD2 sometimes accelerates cancer and other times reverses cancer. Perfection.

We have to look at D, because on the real exam, you MUST evaluate every single answer choice. I’ve missed so many practice problems because I was seduced by the first perfect answer I encountered without giving an honest audit of each and every answer choice. Let’s make the Frankenstein sentence for D:

“D. SUSD2 sometimes accelerates cancer and other times reverses cancer can be explained by the fact that: SUSD2 is a steroid hormone receptor expressed only on the surface of specific tissue types”

There are several reasons this Frankenstein sentence must evaluate to false. One, SUSD2 is not a steroid hormone receptor; it is a transmembrane protein that plays a role in how tumors spread. Two, steroid hormone receptors are usually embedded in the cytosol, where, upon binding with a ligand, usually translocates to the nucleus to throttle genes on or off. The answer choice conflates steroid hormone receptors with peptide hormone receptors, which are expressed on the surfaces of cells because peptides are unable to traverse the lipophilic interior of the plasma membrane unassisted.

Yes, you needed to know that information. No, it wasn’t in the passage. But see how there are multiple pieces of evidence to confirm or deny an answer. D doesn’t answer the question, first of all, and second, contains false statements. If you chose D, ask yourself why you skipped over C. Why was D a more compelling answer to our restated question than C?

Ready for another?

Let’s just rephrase it right now, before we even look at the choices: p53 acts before replication. What comes before replication in the cell cycle? Notice how I omitted fluff like “arrests cell growth upon recognition” because we know the crux of the question is, “do you know the different phases of the cell cycle in order? How do we desingate replication? What’s before replication?”

Now let’s take a look at the choices:

Before reading this passage, you will have already known that the S phase, which is when DNA is replicated, follows the G1 phase, which is a time characterized by cell growth. We need growth because you need two of everything! Two sets of Golgi bodies, two sets of lysososomes, etc. The G1/S checkpoint is crucial because once you pass it, your cell is committed to dividing. Our Frankensteins would then be:

“A. the pre-replication checkpoint occurs from: G0 to G1”

This really doesn’t occur if I’m not mistaken. Following cytokinesis, the cell can elect to enter cell cycle arrest, G0, or continue to G1, where cellular contents, excluding the chromosomes, are duplicated. Toss A.

“B. the pre-replication checkpoint is: G1 to S”

This one is beautiful since it makes sense. Keep B.

“C. the pre-replication checkpoint is: S to G2”

Replication occurred in S, so this doesn’t represent anything pre-replication, so get rid of C.

“D. the pre-replication checkpoint is: G2 to M”

This occurs post-replication because this checkpoint is dedicated to error-checking duplicated chromosomes; eliminate D.

Remember Tip 5 from Part II, we need to drill down to the question stem’s core to make our Frankenstein sentences easy to evaluate. Notice how when we got to absolute essentials of the question stem, we need not bother with the p53 discussion. Those words were just interesting distractors to hide the fact that it’s asking you a straightforward “recall the cell cycle” question.

Let’s return to a data analysis question to get some more practice using this incredible technique developed by Altius Test Prep. Here’s a good softball question:

When I read this, I saw:

SMAD 2/3 ↑ ⟼ senescence ↑ and SUSD2 ↑ ⟼ apoptosis ↓ [is or isn’t] explained by…

Notice how I’ve said “is or isn’t”. That’s because 50% of the answer choices start with “yes” or “no.” The question is asking if the data can reconcile the researcher’s proposals. Take a look:

A. SMAD 2/3 ↑ ⟼ senescence ↑ and SUSD2 ↑ ⟼ apoptosis ↓ is explained by: SMAD 2/3 ↑ ⟼ SUSD2 ↓

Notice how I changed my restated question stem to only include “is explained” to account for the affirmation (“yes”) in the answer choice. Ask yourself if the fact that some proteins (SMAD) lead to senescence while others (SUSD2) block apoptosis is explained by SMAD blocking SUSD2? The problem is the researchers never investigated that direction in the answer choice. In other words, there was never any experiment that resulted in SMAD 2/3 ↑ ⟼ SUSD2 ↓. Recall our passage map:

  • [from Paragraph 4] Senescence ↑ =SMAD 2/3 ↑ ⟼ Proliferation / Apoptosis / Differentiation Regulation ↑
  • Figure 1: SUSD ↓ ⟼ SMAD 2/3 ↓
  • Paragraph 5 + Figure 2:

Expt. 1 — TGFß ↑ ⟼ apoptosis ↑

Expt. 2 — SUSD ↓ ⟼ apoptosis ↑

So if you were to conclude that SMAD 2/3 upregulation leads to underexpression of SUSD2, you would be making an inference that wasn’t ever made in the passage, explicitly or implicit in the data. Drop A.

“B. SMAD 2/3 ↑ ⟼ senescence ↑ and SUSD2 ↑ ⟼ apoptosis ↓ is explained by: SUSD ↓ ⟼ SMAD 2/3 ↓”

First of all, the restated relationship from the answer choice should be familiar because it’s literally from our passage map annotating Figure 1. But we have to see if that relationship is answering the restated question stem. Yes, we saw how the SUSD2 KO led to a statistically significant decline in SMAD2/3 expression, but does that explain why SMAD2/3 is upregulated in senscent cells and overexpression of SUSD2 inhibits apoptosis? Let’s trace the logic. If SUSD2 is blocked, then we know from Expt. 2 that apoptosis increases, and also Aging ↑ ⟼DNA damage↑ ⟼ Apoptosis ↑ OR Senescence ↑. So if your cells are committing apoptosis, there’s no need to accumulate proteins like transcription factors to promote senescence. That’s why it’s reasonable to conclude that if Apoptosis ↑ Senescence ↓, so we invert the relationship from Paragraph 4:

-(Senescence ↑ ⟼ SMAD 2/3 ↑ ⟼ Proliferation / Apoptosis / Differentiation Regulation ↑)

which is actually just like distributive property, where we apply a negative to each arrow:

Senescence -(↑) ⟼ SMAD2/3 -(↑) ⟼ Proliferation / Apoptosis / Differentiation Regulation -(↑)

to get:

Senescence ⟼ SMAD 2/3 ⟼ Proliferation / Apoptosis / Differentiation Regulation

So our final chain of logic is:

SUSD2↓ ⟼ apoptosis ↑ Senescence ⟼ SMAD 2/3 ⟼ Proliferation / Apoptosis / Differentiation Regulation

And this makes sense. B looks reasonable 👍🏽

.“C. SMAD 2/3 ↑ ⟼ senescence ↑ and SUSD2 ↑ ⟼ apoptosis ↓ isn’t valid because in reality: SMAD 2/3 ↑ ⟼ SUSD2 ↓”

Notice how I changed up my restated question stem a bit to make it flow better gramatically. Here, we see the problem with A; namely, that no experiment could help us come up with that inverse relationship between SMAD2/3 and SUSD2. Reject C for the same reason we rejected A.

“D. SMAD 2/3 ↑ ⟼ senescence ↑ and SUSD2 ↑ ⟼ apoptosis ↓ isn’t valid because in reality: SUSD ↓ ⟼ SMAD2/3 ↓”

Notice how B and D are very, very similar. Your job is to determine whether or not the data, which indicated that SUSD ↓ ⟼ SMAD 2/3↓, is reasonable to explain the question stem. You have to realize that the cell can either commit to programmed cell death or enter senescence once the damage is accumulated (Aging ↑ ⟼DNA damage↑ ⟼ Apoptosis ↑ OR Senescence ↑). With this technique of composing Frankenstein sentences, you can easily see how when you take the restated question together with your summarized answer, verification becomes a joke. In the case for D, how does that fact that our SUSD KO leading to less SMAD2/3 actually not explain why senescent cells are brimming with SMAD2/3 whereas SUSD2-filled cells never commit suicide?

I should note that I’m doing these problems “live” on this blog— that is, I’m performing the technique on a passage I’ve never seen before and putting my work and thoughts directly on this blog to prove that it works. I hope this was illustrative for you.

If you have any doubts about this technique, try it out yourself. Also, check back here regularly for more proof that the Altius method works wonders.

Previous posts:

Part I

Part II