Join us as we dissect passage 3 from the Biology/Biochemistry section of the Blueprint diagnostic exam.
We’re joined by George from Blueprint MCAT. Go sign up for a free account where you get access to this half-length diagnostic as well as many other amazing tools and resources. If you would like to follow along on YouTube, go to premed.tv.
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[02:09] MCAT Tips
If you find yourself getting immediately turned off by the content of the passage even before jumping into it, George advises you to block off the noise in your head.
Replace these big ugly names, with like a letter and it becomes a lot less intimidating. It’s just a name. You don’t need to come up with any sort of preconceived notion about the specific disease states or any specific content.'You don't need to focus on the big scary elements. Focus on exactly what's important.' Click To Tweet
George says students need to focus on the MCAT testable stuff, the relationships, the variables, and how things relate to each other. Focus on those buzzwords, like proteins and amino acids. Focus on what you know, and use that to orient yourself towards answering the questions.
Notice how things relate. And for all the fluff in between – you can change the name to whatever you want because it doesn’t matter.
[04:07] Passage 3
A woman began canning her own jams as a hobby. She shared several cans of her jam with her mother for lunch. The next day, her mother was found suffering from blurry vision, difficulty swallowing, and troubled breathing. Given the quick progression of symptoms, associated with the new food, the ER physician suspected botulism poisoning. Due to the danger and toxicity of botulism, tests were performed immediately to determine the best course of treatment.
Focus on the MCAT testable stuff. You may highlight the symptoms. Just remember that there’s something involved with vision, swallowing, and breathing.
[05:14] Paragraph 2
The botulinum toxin is a neurotoxin produced by the bacterium Clostridium botulinum. Botulism is a life-threatening illness in humans, although forms of the toxin are used for various cosmetic and medical procedures. The eight distinct toxin types are designated A to H. The botulinum toxin protein is a two-chain polypeptide with a 100-kDa heavy chain joined by a disulfide bond to a 50-kDa light chain. This light chain is a protease that attacks one of the fusion proteins (SNARE protein) at a neuromuscular junction, preventing vesicles from anchoring to the membrane to release acetylcholine.
Figure 1 Mechanism of neuromuscular blockade by botulinum toxin
Now we’re starting to get into some more cellular-level stuff. We’re continuing along this idea of botulism. We can then highlight “neurotoxin.”
We’re given the idea that the toxin comes in various forms including those used for cosmetic and medical procedures. Then you can highlight MCAT testable words here such as two-chain polypeptide and disulfide bond. You may also want to highlight protease which they refer to as a light chain.
George couldn’t stress enough the importance of relationships. In this case, it says that the light chain prevents the release of acetylcholine. So this is something to probably remember as well.
[08:06] Paragraph 3
Tests employed to detect botulism include brain scans and nerve conduction tests. Toxicity testing of serum specimens, wound tissue cultures, and stool specimen cultures are the best methods for identifying botulism, though they are time-consuming. If the symptoms are diagnosed early, treatment can reduce fatality to less than 20%. A faster way to detect the toxin in humans is using mass spectrometry.
Now we’re getting more into the clinical aspects. This paragraph focuses on the keywords, “detect botulism.” We’re going to run some tests and look for botulism. Then they mention this new technique called mass spectrometry. Also highlight the part where if detected early, it reduces fatality to less than 20%.
[09:46] Question 11
What is a likely neurological symptom caused by the toxin’s effect on acetylcholine release?
Just based on what Botox is used for, it’s to relax muscles, and so it seems the answer here is C. While the other answer choices don’t make sense. That’s why it’s important to be able to make those connections.
That being said, one area that this question could be challenging for people, George adds, is if you read the question too quickly.
Acetylcholine release is associated with muscle contraction which leads to tetanus or muscle spasms. But both answers can’t be correct. And so, the key to this question is “caused by the toxin’s effect on acetylcholine release.”
From our understanding of the passage, the toxin’s effect is to prevent acetylcholine release. In which case, we would prevent muscular contraction just like the actual toxin Botox that’s put on the face where you can’t move it.
Correct Answer: C
[11:10] Question 12
The easiest method to separate the two subunits of the botulinum protein for subsequent analytical purposes would be:
Going back to Paragraph 2, we see the two-chain polypeptide and disulfide bonds. There are two parts to it. One is the heavy chain, which is 100-kDa, and the other is a 50-kDa light chain. There’s a big difference in the size.
And so, the size-exclusion chromatography would be the correct answer based on just chromatography. Chromatography is a technique to separate things. Spectrometry is a technique to measure and classify things.
B – Mass spectrometry was dropped towards the end of the passage. But in this case, it’s a test to detect botulinum, not to separate it. Ultimately, you can figure out the actual size and maybe a little bit of the properties of the compound you’re looking at, but it doesn’t give you any separation in terms of the other separation methods.
Gas vs. Thin-Layer vs. Size-Exclusion
And so, now we’re down to gas chromatography, thin-layer chromatography, and size-exclusion chromatography.
A – Thin-layer chromatography is the classic thing that you’ll see in your organic chemistry lab where you have those silica plates. Then you put a solvent of some sort and then the capillary action comes up based on polarity.
C – Gas chromatography also separates things but as the name suggests, in order to separate it, you have to turn it into a gas. Therefore, you have to vaporize it. But botulinum is a protein and if you try to vaporize a protein, it will be destroyed. And so gas chromatography applies to usually volatile substances like organic solvents.
In this case, size-exclusion chromatography, in the name itself, size-exclusion is based on size chromatography. It’s a separation based on size.'Learning is fun when you know what's going on.'Click To Tweet
George explains that once you know what’s going on, it’s so fun to see everything click together and make sense. You don’t have to think about it or memorize it. It’s like a language where things fit together.
Correct Answer: D
[17:40] Question 13
Which molecule could be used to detect toxin D in the cultures described in the passage?
C.A radiolabeled thymine
We didn’t see any specifics about toxin D, but it is one of the eight distinct types of toxins. We know it is a protein. If we paraphrase it, what can we use to detect some sort of unique protein when it comes to these macromolecules?
B – Phospholipids are classically found in our membrane. There are the polar and nonpolar parts. It’s part of our membranes that are not really used for targeting proteins not used for detecting proteins.
C – Thymine is a nitrogenous base on its own that’s known to interact with proteins.
A & D – The difference between an antibody and an antigen is that an antigen is a protein that is capable of being like a spiked protein or Coronavirus for example. Whereas an antibody sticks to very specific and unique parts of proteins. They stick to the antigens.
In this case, when we’re trying to detect a specific protein, antibodies have a specific complementary site. This is what’s going to stick onto your toxin D. The antibodies will stick onto your antigen or the toxin. You wouldn’t use an antigen to detect another antigen, you use an antibody.
It’s easy to get distracted on the MCAT, that’s why it’s important to paraphrase and distill the question to something simpler. This makes your thought process easier.
Correct Answer: A
[24:56] Question 14
Which of the following is NOT a function of the sympathetic nervous system?
A.Increased heart rate
This is a pseudo discrete question. Sympathetic is fight or flight. Make sure first that this is a NOT question. So A – increased heart rate is a sympathetic response so we cross this out. B – pupillary constriction is also a sympathetic function so we strike that out.
Now, we’re left with C and D. Vasoconstriction is also fight or flight. Vasodilation is relaxation so it’s not a fight or flight response. Hence, C is the correct answer.
Correct Answer: C
[32:12] Question 15
The symptoms of botulinum took 12-18 hours to be observed in the patient. How is this best explained?
A.Growing bacteria produce the toxin.
B.Botulinum toxin is immediately filtered by the kidneys and excreted.
C.Toxins can be absorbed through mucous membranes or respiratory tract.
D.Most of the toxins are polar and they take time to cross the nuclear membrane.
Here it says that there’s a delay in the symptoms. And paraphrasing the question, what causes the delay of the symptoms?
A – Growing bacteria produce the toxin. You start growing them and they start multiplying. It takes time for them to grow.
B – If this is the case that the toxin will be immediately excreted, then you’re not going to be affected by the toxin and there would be no symptoms.
C – This statement is probably true. But the question is, does it apply? Remember, your mouth has mucous membranes. And so if you ate something and it started getting absorbed through the mucous membrane, that logic is that you’d be affected immediately. And there wouldn’t be any delay. So while the statement is true here, it doesn’t apply to this question.
D – Most of the toxins are polar and they take time to cross the nuclear membrane. If we think about polar versus nonpolar things, the classic thing to remember is that for anything to go from extracellular to intracellular, it has to be nonpolar. Examples of nonpolar are your cholesterol and steroid-based hormones. These are the things that are going to come across your membrane and go towards your nucleus. But there is no big toxin that is ever going to cross your membrane and get your nucleus.
Correct Answer: A