USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 3-month-old male infant is brought to the emergency department by his mother because of epistaxis for the last 2 hours. His mother denies any trauma and reports he was sleeping when she noticed the nose bleed. He has a history of severe eczema and has had a prior episode of sinusitis as well as bacterial pneumonia. Physical examination shows active bleeding of the left nasal septum, diffusely dry skin, and multiple, small, red spots measuring 1-2 mm on the lower abdomen. Which of the following is the most likely diagnosis?
Contributors:Charles Davis, MD, Sam Gillespie, BSc, Evan Debevec-McKenney, Samantha McBundy, MFA, CMI
So, one by one, there’s eczema, also called atopic dermatitis, which is characterized by dry red patches arising on the skin.
There’s a type of thrombocytopenia called microthrombocytopenia because not only are there very few platelets, but the platelets are also small in size.
And there’s a problem with the immune system that leads to repeated infections.
All of the hematopoietic cells, which are cells in the bone marrow, produce Wiskott-Aldrich syndrome protein, or WASp for short.
There’s also a gene - called the WIPF1 gene, which encodes a protein called WAS/WASL-interacting protein family member 1, which helps stabilize Wiskott-Aldrich protein.
So WASp, aside from having a really long name that shortens down to the name of a scary flying insect - helps to reorganize the cell’s cytoskeleton, and therefore its overall shape.
The cytoskeleton can change by either adding to or removing actin proteins from the end of an actin chain.
The chain grows longer in the direction that a cell wants to move and shortens on the side that a cell wants to move away from.
This helps with various cellular activities like phagocytosis and cellular division.
Platelets specifically rely on this functionality, because they originate from large precursor cells called megakaryocytes.
This megakaryocyte has many long arms - like a squid - and the cytoskeleton changes shape so that these arms can detach to form cellular fragments called platelets.
The platelets then go off to form clots at damaged sites in the blood vessels, to stop bleeding.
Another cell type are the T-cells, which are a type of immune cell, also rely on the cytoskeleton being able to change shape.
When they encounter a pathogen, T-cells form pseudopods or false legs that reach out and synapse or communicate with other cells.
Think of it like they’re shaking hands to exchange information.
Next up are the cytotoxic T-cells and natural killer cells, which also reorganize their cytoskeleton to form an immunological synapse with various body cells to do surveillance, and find out if they’re healthy or if they’re infected or cancerous.
If an unhealthy cell is discovered, the immune cells make that unhealthy cell undergo apoptosis, or programmed cell death.
Together, the T-cells, B-cells, and natural killer cells protect the body from pathogens as well as cancer.
T-regs also rely on reorganizing their cytoskeleton to function normally. Finally, there are the phagocytic cells like monocytes, macrophages, and dendritic cells, which form small foot processes to make their way towards cytokines.
These phagocytic cells are like little bloodhounds and following a cytokine trail.
These cells also perform phagocytosis, to swallow up debris, dead cells, and bacteria, so that it can be processed and destroyed.
In Wiskott-Aldrich syndrome, a mutation in the gene results in a Wiskott-Aldrich protein that can’t function normally.
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