AAMC SB #24 B/B

[rabbott1971]

This passage is one of the kind that sort of paralyze me and I start bogging down hard. This is probably due to my own insufficient grasp of these things, but I would be interested if someone has sort of a mental thought process to how to solve this in 100 seconds or so. This just seems way beyond things I've learned in school. Thanks!

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[aldol16]

I just skimmed (a bit drunk since it's the weekend of the Fourth )so I might be wrong on certain aspects of the experiment, but from Fig. 1, you can see that STRA6 helps retinol bind. According to the passage, STRA6 is a receptor that is specific to RBP. In other words, it only helps in the presence of RBP. So the cells that are not transfected with STRA6 or transfected in the absence of RBP should show little to no binding. siRNA targeting STRA6 would only affect RBP-bound retinol, since STRA6 is a receptor that is specific to RBP. Therefore, it should reduce the retinol in RBP only. So B fulfills all of these - STRA6-transfected ones have increased binding in the presence of RBP (but not BSA) that that is reversed in the RBP group that was treated with the STRA6-inhibiting siRNA.

Rule out C because untransfected cells there all seem to bind retinol differently when there shouldn't be any difference and they should not be binding much retinol. Untransfected cells are your negative controls here. Rule out A because it says specifically in the passage that the siRNA works on STRA6 in the presence of RBP so if you block a receptor that helps retinol binding (STRA6), then you would expect reduced binding. Finally, rule out B because you wouldn't expect STRA6 to help at all without RBP - it's the receptor for RBP.

 

[rabbott1971]

Thanks, your analysis tracks the answer. This kind of passage is really hard for me to keep in my head what is going on, and what it is talking about. I am in danger of literally ending up reading for three minutes and getting very little comprehension. I went through it again and tried to make a little note page. I might start trying to do this with the really hard passages, just to keep things straight in my head. Is this a technique some people use? I can see it being time-consuming, but at the same time, it seems better than reading through and then going "what did I just read?"

My sketch probably reflects misunderstanding of the passage, if you have time for a correction or two, please blast away. When I got further along it occurred to me to wonder why the LRAT and CRBP have such an impact on uptake of retinol, when they are described as facilitating processes inside the cell (storage or conversion).

In Figure 2, it is tracking the continued existence of holo, which would tell you how much retinol did NOT get taken up? Why do they use the word "release" in Figure 2 instead of uptake? Thanks Aldol.

 

[aldol16]

Draw whatever you have to to help you visualize the passage on the MCAT. I actually did use shorthand on the MCAT to help me keep track of all the variable so it's not uncommon at all - actually it's recommended because now instead of having to read over the passage again to find the relevant parts, you can basically refer to your diagram for the important relationships when you're answering questions.

In figure 2, they are measuring the fluorescence of the holo-RBP (retinol+RBP). They say in the passage that holo-RBP fluoresces more than non-bound retinol. So yes, they are basically measuring how much retinol is still bound to RBP. As to why they use the "release" terminology, it's because they're interested in the process of retinol release from holo-RBP. Measuring how much is still bound is still a direct measurement of how much is released if you know how much was bound to begin with.

LRAT and CRBP-1 likely have an effect on retinol uptake because the system is composed of interacting parts. You can think of it as an equilibrium. holo-RBP ---> STRA6 with retinol ---> retinyl esters. If you remove retinyl esters to store them, equilibrium shifts to the right. If you make a lot of retinyl esters, which subsequently get stored, you also shift equilibrium to the right.

 

[rabbott1971]

Very helpful, thank you!

 

[DPTinthemaking15]

Very helpful, thank you!

This is the same passage I just tackled and got destroyed by it. What was your course of action to tackle these types of passages on the MCAT? I made a mini-note page while reading, but I reached the end and realized I had no clue what just happened lol.

 

[rabbott1971]

This is the same passage I just tackled and got destroyed by it. What was your course of action to tackle these types of passages on the MCAT? I made a mini-note page while reading, but I reached the end and realized I had no clue what just happened lol.

I wish I had good advice, I did OK on the MCAT (511) but I only got 125 on the bio portion. I think it is by far the hardest part. Try to know a lot of stuff! Kaplan does recommend making notes, but I guess now you'll have to use those wet erase board things. Stay calm and try to reason it out. Think about what you know about the topic, and try to apply it to the question. There are little small things that make answer choices wrong, try to find them. Not a lot of help, but in general, that's kind of how you have to pick through unless you're one of the super geniuses who knows everything. Good luck.