NBME 11 Questions

[Enzymes]

Hey friends, I have a couple of questions that I ran into during my NBME 11 today. Any insight about anything is very appreciated.

1. In a positive PPD induration, the predominant cell is what? The answer was macrophages, with the main distractor being CD8s. I know that a Type IV Hypersensitivity rxn activates T cells AND macrophages, but would CD4 T cells be correct? I always thought T cells were the main responders that cause the induration in a positive PPD test.

2. I got a transudate vs. exudate question, and I can't tell what the correct answer is. Basically SG: 1. 02, many neutrophils, and cellular debris. I thought the answer was exudate, but the answer key posted by some random person online says transudate. I is that a mistake? Any thoughts on this one?

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[thehundredthone]

1. CD4 T cells do not act at the site of the reaction, they merely signal to activate the effector cells.

2. I don't have access to NBME 11 right now, what fluid was it? They usually don't bother with specific gravity unless it's urine.

 

[Apoplexy__]

2. I got a transudate vs. exudate question, and I can't tell what the correct answer is. Basically SG: 1. 02, many neutrophils, and cellular debris. I thought the answer was exudate, but the answer key posted by some random person online says transudate. I is that a mistake? Any thoughts on this one?

This was a CHF patient right? I was also fooled by the cellular debris, instantly jumping to exudate. Thing is, the #1 most important thing is the specific gravity/protein levels. Wasn't it significantly lower than 1.02?

Bottom line: You can have cells in transudate -- otherwise there would be no RBC's for alveolar macrophages to phagocytose so they could be called "heart-failure cells". Look at the specific gravity + context of question.

 

[Enzymes]

This was a CHF patient right? I was also fooled by the cellular debris, instantly jumping to exudate. Thing is, the #1 most important thing is the specific gravity/protein levels. Wasn't it significantly lower than 1.02?

Bottom line: You can have cells in transudate -- otherwise there would be no RBC's for alveolar macrophages to phagocytose so they could be called "heart-failure cells". Look at the specific gravity + context of question.

No, that was a different question. Generally with CHF, you are dealing with a transudate because it is not due to inflammation, but rather back up of fluid and increased hydrostatic pressure. I am talking about #22 in the second block at this link: http://www.scribd.com/doc/110644085/NBME-11-With-Answers

 

[001100010010011]

1. There is a similar question in UWorld asking about what you would see in a PPD. For this question, CD4 cells signal macrophages to the site.

2. I picked that answer because of the hydrostatic P increase, leading to a transudate.

 

[Enzymes]

1. CD4 T cells do not act at the site of the reaction, they merely signal to activate the effector cells.

2. I don't have access to NBME 11 right now, what fluid was it? They usually don't bother with specific gravity unless it's urine.

Thanks but here is my confusion. In contact dermatitis (also a Type 4 HS), UWorld claimed that the main effector cells were CD8 T cells (which were previously sensitized on first exposure to the allergen). Does this mean PPD is acting via a different mechanism where CD4 and macrophages are more important? I was confused by the difference between PPD and contact dermatitis.

 

[001100010010011]

I remember that question--definitely the one I was referencing. Do you have the ID for it? I think there was something specific to the question like "what causes the damage" or something along those lines that made you pick CD8.

 

[Enzymes]

I remember that question--definitely the one I was referencing. Do you have the ID for it? I think there was something specific to the question like "what causes the damage" or something along those lines that made you pick CD8.

ID 1133. Monocytes was the other choice I was considering on that one. Generally the answer is obvious in Type IV (they aren't trying to trick you between macrophages and CD4 or CD8, but it occasionally comes up. Any thoughts about how to sort this all out?

Also, I am not referring to that CHF patient with the transudate vs. exudate question. They explicitly said the SG was 1.020, which is high. Isn't that exudate? I am only confused because the online answer key seems to say transudate.

 

[Apoplexy__]

No, that was a different question. Generally with CHF, you are dealing with a transudate because it is not due to inflammation, but rather back up of fluid and increased hydrostatic pressure. I am talking about #22 in the second block at this link: http://www.scribd.com/doc/110644085/NBME-11-With-Answers

Ah, my mistake. It's exudate, I remember it being a mistake when I was going through that answer key. I got extended feedback, and put "exudate" and it didn't include that question in my incorrects. You're right, everything about the question screams exudate.

 

[Enzymes]

Ok, think I figured it out for those interested. PPD is TB, which induces granulomatous inflammation. So macrophages are more important b/c you need to wall off TB. In contact dermatitis, it is the same process, but granulomas are not forming, so T lymphocytes become more important. Let me know if that makes sense or not. Thanks for the discussion!