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NBME 21 Answers

nbme21/Block 4/Question#15

A 5-year-old boy is brought to the physician for a ...

Lack of choroidal pigment deposition in the macula

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submitted by henoch280(15),

@temmy. This question tests our knowledge on albinism which is normally a tyrosinase deficiency disease but the vignette states that the boy's albinism is caused by a genetic mutation in the TYRP1 gene which is shown in the biochemical pathway. A gene that helps in the synthesis of Eumelanin.

Now you have to understand that all precursors before that gene is the pathway would still be available if not increased which make 2 of the options in the question wrong.

you also have to understand this: (Eu)melanin = (normo)melanin i.e normal melanin which is protective to the skin, decreases reactive oxygen species and gives the dark pigments to the iris, choroid, skin, hair e.t.c. while (feo)melanin = (fake)melanin i.e pheomelanin, the one present in our patient here which is less protective again the uv rays, cannot pigment and cannot decrease ROS generated in the skin. i hope this helps





Proccess of elimination for this one. Two you can eliminate immediately just from looking at the biochemical pathway chart. The other two required knowledge that eumelanin is more protective than pheomelanin (this is why redheads burn more easily). Because pheomelanin is less protective, there would be more not less ROS from sunlight.

temmy  i don't understand this at all. i am completely blank...please help +1  
henoch280  @temmy. This question tests our knowledge on albinism which is normally a tyrosinase deficiency disease but the vignette states that the boy's albinism is caused by a genetic mutation in the TYRP1 gene which is shown in the biochemical pathway. A gene that helps in the synthesis of Eumelanin. Now you have to understand that all precursors before that gene is the pathway would still be available if not increased which make 2 of the options in the question wrong. you also have to understand this: (Eu)melanin = (normo)melanin i.e normal melanin which is protective to the skin, decreases reactive oxygen species and gives the dark pigments to the iris, choroid, skin, hair e.t.c. while (feo)melanin = (fake)melanin i.e pheomelanin, the one present in our patient here which is less protective again the uv rays, cannot pigment and cannot decrease ROS generated in the skin. i hope this helps +8  
eacv  @henoch280 thank u very much! I got it right by luck but now i do understand :D +  




 +1  upvote downvote
submitted by sweetmed(41),

Eumelanin also helps the choroid limit uncontrolled reflection within the eye that would potentially result in the perception of confusing image





 +0  upvote downvote
submitted by sahusema(29),

I cut open an eyeball once and I was surprised how dense black the material inside was. That plus process of elimination brought me to the right answer