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Retired NBME 15 Answers

nbme15/Block 3/Question#30 (reveal difficulty score)
A 13-year-old boy is scheduled to receive ...
T-lymphocyte thymocytes localized to the thymic cortex ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—andro(269)
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First question : They tell us its a lymphocytic leukemia

Second question : Deciphering whether it is a B cell or T cell.

The cells have no surface bound Ig M ,Ig G or cytoplasmic chains for these antibodies , but have a rearranged gene segment( meaning active ) for the T cell Receptor . So the Cells are T Cells

Third question : Now that we know that they are T Cells , we want to know whether they are CD4 or CD 8 positive

The cells in the clinical vignette lack both CD4 and CD 8 ( double negative ) According to how T cells normally develop - They start of as being double negative ( i.e not expressing CD4 or CD 8) in the subcapsular region and progressively become
- positive for both CD 4 and CD 8 in the cortex
- and eventually acquire the TCR and CD3 by the time they get to the medulla

These cells therefore must T lymphocyte thymocytes localised to the cortex

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i_hate_it_here  FA2020 pg: 102 +1
dawgtor  Don't cells in the thymic cortex express both CD4 and CD8? That threw me off a bit. I went with activated cytolytic effector T cells because i thought they would bind MHC I. +
makingstrides  Yeah, that's what I chose too @dawgtor. But I think when the cells are made in the bone marrow, they don't have the MHC classes/ TCR either. When they get into the thymic cortex, they gain those and undergo positive selection. This is from FA2020 page 102. Correct me if I am wrong. +1
victorlt14  The thymocyte (T cell precursor, not T cell quite yet) gets in the subcapsular zone of the cortex with no CD4/CD8. It took me longer to be sure it was a cell of T lineage though. Looking back I think it should've been rather easy to recognise that since the cell line has a T cell receptor it is of T cell lineage. +



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submitted by letitb(4)
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Also the rearrangement of the t-lymphocyte receptor beta chain gene d and j segments they are referring to is seen in PRE-T CELL.

https://www.google.com/url?sa=i&url=https%3A%2F%2Fusmle287.wordpress.com%2F2012%2F10%2F23%2Flymphocyte-maturation%2F&psig=AOvVaw1QRew1yQvBGT3wZnRhPWfz&ust=1598659306513000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCOC32JHMvOsCFQAAAAAdAAAAABAr

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