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 +7  (nbme22#5)

Short answer: All genes are present in both B and T cells, but they rearrange and express receptors that are specific to their lineage. Their are independent VDJ regions for B cell receptor generation and T cell receptor generation! This wiki article sheds light on this:


I too tragically missed this.. but it is actually so simple that we are missing the forest through the trees -- While both B and T cells undergo VDJ rearrangement, the V, D, and J genes are DIFFERENT GENES despite being organized and therefore classified in a similar fashion due to their shared need to generate diverse structures. To make matters more confusing, both sets of VDJ genes are on chromosome 14.

This is why the NBME specified that they are tagging THE SAME J REGION which is that of the B cell receptor gene J region, not the T cell receptor gene J-beta region (see wiki article). The B cell receptor gene is not rearranged in a T cell, if it was, T cells would have antibodies!

redstrat  This. So much this!

Subcomments ...

submitted by smc213(95),

To be completely clear!

This patient has Cystinosis a rare autosomal recessive lysosomal storage disorder and most common cause of Fanconi syndrome in children. Cystinosis is systemic and leads to cystine crystal deposits in cells and tissues throughout the body.

Although Wilsons disease can lead to FS, the crystals in the corneas does not correlate with Wilsons disease.
More info: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4841061/

highyieldboardswards  Thank you! You are a legend for figuring this out! +  
paulkarr  Appreciate you. +  
drzed  And even if it was Wilson disease, it would have the exact same consequence leading to Fanconi syndrome. +1  
abhishek021196  Fanconi syndrome Generalized reabsorption defect in PCT = Increased excretion of amino acids, glucose, HCO 3 – , and PO 4 3– , and all substances reabsorbed by the PCT May lead to metabolic acidosis (proximal RTA), hypophosphatemia, osteopenia Hereditary defects (eg, Wilson disease, tyrosinemia, glycogen storage disease), ischemia, multiple myeloma, nephrotoxins/drugs (eg, ifosfamide, cisplatin), lead poisoning. Polyuria, renal tubular acidosis type II, growth failure, electrolyte imbalances, hypophosphatemic rickets = Fanconi syndrome (multiple combined dysfunction of the proximal convoluted tubule). +  

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