Official Step 1 High Yield Concepts Thread

[Transposony]

Let's discuss our doubts/offer clarifications about mechanisms/concepts for Step 1

ASK ANY QUESTIONS here.

To kick start the thread here is something I didn't know:

1. Penicillin-binding proteins (PBPs) are actually enzymes (transpeptidases & carboxypeptidases) which cross-link peptidoglycan. Penicillins binds to these enzymes and inactivating them thereby preventing cross-linkiing of peptidoglycan.

2. Periplasmic space (Gram -ve) contain proteins which functions in cellular processes (transport, degradation, and motility). One of the enzyme is β-lactamase which degrades penicillins before they get into the cell cytoplasm.
It is also the place where toxins harmful to bacteria e.g. antibiotics are processed, before being pumped out of cells by efflux transporters (mechanism of resistance).

There are three excellent threads which you may find useful:

List of Stereotypes

Complicated Concepts Thread

USMLE images

---

Members don't see this ad.

 

[Transposony]

Reason for the difference in blood lactate levels between Von Gierke and Cori disease?

Normal blood lactate levels in Cori disease since Gluconeogenesis is intact.

 

[Transposony]

I believe gatifloxacin also has activity against anaerobes.

That's a moot point since Gatifloxacin has been removed from the market as it causes dysglycemia.
So, they are unlikely to ask about it one the test.

 

[seminoma]

That's a moot point since Gatifloxacin has been removed from the market as it causes dysglycemia.
So, they are unlikely to ask about it one the test.

Idk I've heard about muromonab and pergolide showing up. Both of those have been off the market.

 

[Transposony]

Idk I've heard about muromonab and pergolide showing up. Both of those have been off the market.

I agree with you since it is USMLE.
But, there is a difference.
Muromonab was voluntarily withdrawn from the United States market due to decreased utilization but still used in rest of the world.
Pergolide was withdrawn from the U.S. market but still used in rest of the world.
In contrast, Gatifloxacin has been removed from the market and it is no longer being manufactured.

 

[faisal 2000]

Great.
To summarize the differentiating feature are:
Atropine poisoning --> dry skin (Muscarinic block as sweating is sympathetic muscarinic mediated)
TCA overdose --> dry skin with arrhythmia and hypotension (Muscarinic and alpha 1 block)
Amphetamine toxicity --> warm and sweaty skin (as sweating is sympathetic muscarinic mediated)
They all have s/s of sympathetic over-activity.

- atropine poisoning treated by physostigmine
- tca overdose treated by sodium bicarbonate (NaHCO3)
-amphetamine toxicity treated by ??????
-opioid poisoning treated by naltrexone (long acting)

 

[faisal 2000]

Great.
Atropine poisoning --> dry skin (Muscarinic block as sweating is sympathetic muscarinic mediated)
TCA overdose --> dry skin with arrhythmia and hypotension (Muscarinic and alpha 1 block)
Amphetamine toxicity --> warm and sweaty skin (as sweating is sympathetic muscarinic mediated)
They all have s/s of sympathetic over-activity.

benzodiazepine overdoses result in mydriasis, tachycardia, disruption of sleep pattern
treated by Flumazenil

 

[faisal 2000]

what is the mechanism to treat hyperkinetic children

 

[ewax]

amphetamine toxicity treated by ??????

acidifying the urine

 

[seminoma]

Don't know the answer to this, so hopefully one of you can provide a fairly comprehensive explanation.

Why does thiamine deficiency cause vasodilation? I know that thiamine deficiency reduces ATP synthesis, but why does this dilate arterioles?

I agree with you since it is USMLE.
But, there is a difference.
Muromonab was voluntarily withdrawn from the United States market due to decreased utilization but still used in rest of the world.
Pergolide was withdrawn from the U.S. market but still used in rest of the world.
In contrast, Gatifloxacin has been removed from the market and it is no longer being manufactured.

Wow, did not know!

 

[Transposony]

Don't know the answer to this, so hopefully one of you can provide a fairly comprehensive explanation.

Why does thiamine deficiency cause vasodilation? I know that thiamine deficiency reduces ATP synthesis, but why does this dilate arterioles?

Nobody knows for sure so they can't ask it on the test.
However, it's been postulated that since AMP can't be converted to ATP, it gets converted to adenosine which accumulates intracellulary and gets pumped out into plasma causing vasodilation.

 

[Transposony]

Name two bacteria whose capsule makes them non-immunogenic and why?

 

[Transposony]

Which is the Niacin producing Gram+, Catalase+ bacteria ?

 

[Transposony]

Can Tzanck test be used to diagnose HSV-1, HSV-2 , VZV or all three?

 

[dfib slim]

Can Tzanck test be used to diagnose HSV-1, HSV-2 , VZV or all three?

I'd guess yes since they are all a part of the herpes family

 

[syoung]

Name two bacteria whose capsule makes them non-immunogenic and why?

Strep pneumo
H influenzae
Neisseria
GBS
klebsiella
salmonella typhi
psuedomonas aeruginosa
cryptococcus neoformans (fungi)

SHiN Kills Some Patients with Capsules
Strep, H influenza, klebsiella, salmonella, pseudomonas, cryptococcus

others:
e. coli, bordatella, strep pyogenes, anthrax

 

[dfib slim]

Name a gram + organism that produces endotoxin

 

[dfib slim]

Which is the Niacin producing Gram+, Catalase+ bacteria ?

Mycobacterium tuberculosis

 

[Transposony]

Strep pneumo
H influenzae
Neisseria
GBS
klebsiella
salmonella typhi
psuedomonas aeruginosa
cryptococcus neoformans (fungi)

SHiN Kills Some Patients with Capsules
Strep, H influenza, klebsiella, salmonella, pseudomonas, cryptococcus

others:
e. coli, bordatella, strep pyogenes, anthrax

I think you misread the question.

 

[Transposony]

Name a gram + organism that produces endotoxin

No idea since endotoxins are part of the outer membrane and only Gram-ve bacteria have the outer membrane.

 

[guspilgrim]

Name a gram + organism that produces endotoxin

Listeria

 

[dfib slim]

No idea since endotoxins are part of the outer membrane and only Gram-ve bacteria have the outer membrane.

Listeria is the exception

 

[syoung]

I think you misread the question.

Aren't they immunogenic, esp if you have no spleen, are young w/out a developed immune system? Because of their polysaccharide capsule?

 

[seminoma]

Aren't they immunogenic, esp if you have no spleen, are young w/out a developed immune system? Because of their polysaccharide capsule?

I think he's talking about GAS (hyaluronic acid capsule) and B-type N.meningititis.. or however you spell that.

 

[Transposony]

Aren't they immunogenic, esp if you have no spleen, are young w/out a developed immune system? Because of their polysaccharide capsule?

But I was asking about non-immunogenic

 

[syoung]

But I was asking about non-immunogenic

Err I meant to say "Aren't they non-immunogenic"...

 

[Transposony]

Err I meant to say "Aren't they non-immunogenic"...

I think we are going in circles here.
As Seminoma said, Streptococcus pyogenes due to hyaluronic acid and type b Neisseria meningitidis due to sialic acid.
These make their capsule non-immunogenic and it is the reason why there is no vaccine against type b Neisseria meningitidis.

 

[Transposony]

Listeria is the exception

Source?
This is what I found:
http://en.wikipedia.org/wiki/Listeria_monocytogenes

An early study suggested that L. monocytogenes is unique among Gram-positive bacteria in that it might possess lipopolysaccharide, which serves as an endotoxin. Later it was found to not be a true endotoxin.

 

[dfib slim]

FA 2014

Saw that wiki article but it didn't link the source disproving it. Since it was classified at one time as having LPS and maybe disproved later unsure if that info made it back to the USMLE.

Original article
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC414241/

Others
http://www.medbullets.com/step1-microbiology/4038/listeria-monocytogenes
http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6968.1981.tb06199.x/abstract

 

[Transposony]

Sodium-channel blockade:
IC > IA > IB

Increasing the ERP:
IA > IC > IB (decreases)

http://www.cvpharmacology.com/antiarrhy/sodium-blockers.htm

http://www.pharmacology2000.com/Cardio/antiarr/antiarrtable.htm

 

[dfib slim]

Electrolyte abnormality with Legionella?

 

[Transposony]

Electrolyte abnormality with Legionella?

SIADH>>>>>>Hyponatremia>>>>>>>Confusion

 

[Transposony]

Mechanism of flatulence in Giardiasis?


Dimorphic Fungii Mnemonic:
Body Heat Changes Shape
Blasto, Histo, Coccidio, Sporothrix

 

[Transposony]

Mechanism of action of Chloroquine and it's limitations ?

Blood disorders which prevent against Malaria?

 

[Transposony]

Organism?

 

[Transposony]

South american with adult-onset epilepsy ?

 

[Mr. Mojo Risin]

Hm, interesting find about L. monocytogenes. That has always been one of the many random factoids I've committed to memory for micro (FA/Microcards). FA 2015 has it in there.

 

[Transposony]

Difference between Carcinoid disease and syndrome ?

Vitamin deficiency associated with pheochromocytoma?

 

[ALUCSD18]

Hm, interesting find about L. monocytogenes. That has always been one of the many random factoids I've committed to memory for micro (FA/Microcards). FA 2015 has it in there.

What are you referring to? Thanks.

 

[Mr. Mojo Risin]

What are you referring to? Thanks.

In reference to the commonly cited characteristic of Listeria monocytogenes as being the only gram + to produce endotoxin.

 

[syoung]

Mechanism of action of Chloroquine and it's limitations ?

Blood disorders which prevent against Malaria?

hetero sickle cell

 

[Transposony]

hetero sickle cell

What about thalassemia?

 

[dfib slim]

What about thalassemia?

And hereditary spherocytosis. Anything that shortens the life of the RBCs will yield protection.

 

[dfib slim]

Difference between Carcinoid disease and syndrome ?

Vitamin deficiency associated with pheochromocytoma?

Isn't it carcinoid disease when you have a GI tumor causing increased release of serotonin with diarrhea and syndrome when that tumor metastasizes to the liver and you get right heart valvular fibrosis?

Haven't learned about pheo yet. Maybe vitamin D since cholesterol is the precursor?

 

[Mr. Mojo Risin]

And hereditary spherocytosis. Anything that shortens the life of the RBCs will yield protection.

+ G6PD def.

 

[dfib slim]

+ G6PD def.

What about paroxysmal nocturnal hemoglobinuria?

 

[Mr. Mojo Risin]

What about paroxysmal nocturnal hemoglobinuria?

I haven't heard anything about PNH and malaria, so I'm not sure. It seems plausible due to the hemolysis.

 

[Transposony]

Isn't it carcinoid disease when you have a GI tumor causing increased release of serotonin with diarrhea and syndrome when that tumor metastasizes to the liver and you get right heart valvular fibrosis?

Haven't learned about pheo yet. Maybe vitamin D since cholesterol is the precursor?

I meant Carcinoid tumor v/s Carcinoid syndrome.
GI tract carcinoid tumors release serotonin into the hepatic portal circulation which usually undergoes hepatic degradation and therefore does not produce symptoms. However, carcinoid tumors like ovarian which drain into systemic circulation will still cause symptoms.
Carcinoid syndrome occurs when the tumor is so advanced that it overwhelms the liver's ability to metabolize the released serotonin, usually due to metastasis.
It is aka malignant carcinoid.

Vitamin C deficiency is associated with pheochromocytoma since Vit C is a necessary co-factor for dopamine β-hydroxylase, which converts dopamine to NE.


It is similar to Niacin deficiency occuring in carcinoid syndrome as a result of increased tryptophan metabolism into serotonin (tryptophan is diverted to form 5-hydroxytryptophan).

 

[dfib slim]

South american with adult-onset epilepsy ?

Chagas?

 

[Transposony]

What about paroxysmal nocturnal hemoglobinuria?

Someone prove me wrong but unlikely to be PNH since there in nothing inherently wrong with the RBCs in terms of function.
Malaria parasite uses RBC's resources to multiply. It chooses RBCs since it has high O2 demand.

http://www.ncbi.nlm.nih.gov/pubmed/20655368
http://www.usfca.edu/fac-staff/dever/health2.pdf
http://en.wikipedia.org/wiki/Genetic_resistance_to_malaria

 

[Transposony]

Chagas?

The diagnosis of neurocysticercosis should be suspected in all visitors and immigrants from Asia, South America, and Central America who have adult-onset epilepsy.
http://www.nejm.org/doi/full/10.1056/NEJMc070837