USMLE Step 1 Question
Hey this is Dr K from imedicalschool and today we are going to do a USMLE step one practice
question.
Lets say there is an 84 year old male with a past medical history of hypertension, hyperlipidemia
and progressive who dementia passes away at home.
He undergoes an autopsy to identify the cause of his dementia.
Of note no cogwheel rigidity, micrognathia, movement abnormalities or tremor was noted
prior to his passing.
What are the most likely histological findings on brain biopsy?
Is the answer A. Multinucleated giant cells
B. Lewy bodies C. Neurofibrilliary tangles
D. Broad based budding yeast E. Prions
Take some time and pause here to think it over.
Otherwise lets look at the answer choices to find out the correct answer.
There are some key points to note from the stem of the question.
Remember to always think of common things first and when answering a question to not
assume facts that are not given in the question stem.
From the stem of this question we know we have a patient who has progressive dementia
meaning his dementia did not come on suddenly.
Infectious causes of dementia tend to be more rapid in nature and are accompanied by infectious
symptoms such as fever, chills, cough, or gastrointestinal symptoms.
The question clearly points out there is no cogwheel rigidity which is a sign of Parkinson's
disease where movement of a parkinson's patient's extremity is difficult.
In addition there is no evidence of micrognathia which is when a parkinson's patient writes
very small as a result of their disease.
Lack of these movement disorders a rule out Parkinson's disease and similar movement disorders
as a cays of his dementia.
Given these findings lets proceed to look at the answers.
Our first potential choice is multinucleated giant cells.
As the name sounds multinucleated giant cells are large cells that have more than one nucleus.
They tend to arise during an active mycobacterium infection.
These giant cells will eventually form the granulomas seen in longstanding tuberculosis
infection.
Multinucleated giant cells may also be seen in Langerhans cell histiocytosis.
In the end they are associated with areas of inflammation.
Here we have a picture demonstrating these multinucleate giant cells.
Given infection is not a high consideration here since there are no infectious symptoms
or risky exposures this makes A unlikely especially since TB meningitis would need cause a progressive
dementia.
Choice B is lewy bodies.
Lewy bodies are associated with a type of dementia called lewy body dementia.
Lewy body dementia is the second most common cause of dementia after Alzheimer's disease.
Lewy body dementia is associated with fluctuations in cognition, visual hallucinations, and parkinsonian
symptoms.
The parkinsonian symptoms include limb rigidity, gait disturbances, and difficulty initiating
movements, otherwise known as, akinesia.
In this picture we have a depiction of a Lewy body seen as a small sphere in the cytoplasm
of the cell surrounded by brown pigmented material.
Given our patient in this stem has no Parkinson symptoms with dementia makes Lewy body dementia
unlikely.
Asa a result we would not expect Lewy bodies present on histology.
Choic C is Neurofibrilliary tangles.
These tangles are phosphorylated tau proteins that have accumalated in the neurocotical
regions of the brain leading to Alzheimer's dementia.
The tangles appear as wiffs of protein collections, almost looking like a candle flame in the
neuron as depicted in this picture.
Alzheimer's is a progressive dementia and is the most common cause of dementia in the
elderly.
Alzheimer's is characeterized by loss of recent memory including names and dates.
No movement disorders are seen in Alzheimer's and it is a progressive dementia.
Though this choice fits very well with a progressive noninfectious dementia let's make sure none
of the other choices fit well.
In Choice D we have broad based budding yeast.
The description of these yeast indicates a blastomycoces infection.
Blastomycosis infection is unlikely to cause a chronic progressive dementia.
The description of how yeast appear microscopically is very important.
For example if a question says there is appearance of spaghetti and meatballs on a skin biopsy
think times versicolor.
Also if a fruity body is described with 45 degree spectate hyphae think aspergillus.
These are a few examples.
Check out the link in the description below to find a handout about the description of
various fungi.
Let me know if the handout helps.
This brings us to our last choice.
Choice E is Prions.
Prions are abnormally folded proteins that deposit within the brain parenchyma leading
to cell deAth.
Prions lead to diseases like kuru or creutzfled-Jakob disease.
Prions were made famous because these are infectious protein chains that are known to
cause mad cow disease in cows.
CJ disease presents as a dementia and generally patients have corticospinal tract involve
my causing hypereeflexia, babinski sign, and spasticity.
Given our patient does not have these symptoms this makes prion disease unlikely.
In the end it looks like this patient had Alzheimer's disease but it is important to
keep a broad differential and learn how to narrow your differential based on the clinical
presentation.
I hope you enjoyed this video and the handout below.
If you did please give this video a like.
If you have any comments or questions place them in the comments section down below.
Share this video with your friends on twitter, Facebook and Google +.
You can follow us on twitter at imedschool and on Facebook and Google + at imedicalschool.
Most importantly subscribe this is Dr K from imediclschool and I will see you next time.