WBR0977

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Author [[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::A scientist studies the expression and enzymatic activities of aldose reductase and sorbitol dehydrogenase by western blotting and immunohistochemistry. The study was conducted on different samples of rat cells derived from different organs. In one of the samples, the expression of only aldose reductase was positive, whereas sorbitol dehydrogenase was absent. From which of the following organs does this sample of cells most likely derive from?]]
Answer A AnswerA::Liver
Answer A Explanation [[AnswerAExp::Sorbitol dehydrogenase is present in the liver.]]
Answer B AnswerB::Retina
Answer B Explanation [[AnswerBExp::Sorbitol dehydrogenase is absent in the retina. Thus, the sample of cells lacking sorbitol dehydrogenase in the experiment is likely derived from the retina.]]
Answer C AnswerC::Ovaries
Answer C Explanation [[AnswerCExp::Sorbitol dehydrogenase is present in the ovaries.]]
Answer D AnswerD::Seminal vesicles
Answer D Explanation [[AnswerDExp::Sorbitol dehydrogenase is present in the seminal vesicles.]]
Answer E AnswerE::Lens
Answer E Explanation [[AnswerEExp::Sorbitol dehydrogenase is present in the lens.]]
Right Answer RightAnswer::B
Explanation [[Explanation::Glucose is converted to sorbitol by the action of aldose reductase, then sorbitol is converted to fructose by the action of sorbitol dehydrogenase. Sorbitol dehydrogenase is present in the seminal vesicles, ovaries, liver and retina; however, it lacks in retina, shwann cells and kidneys. Therefore, when glucose levels is chronically elevated, as seen in the case of poorly controlled diabetes, glucose gets converted to sorbitol leading to osmotic damage in cells lacking sorbitol dehydrogenease. The complications of this osmotic damage include retinopathy and nephropathy that occur in diabetics. Shown below is a diagram depicting sorbitol metabolism.

File:Sorbitol pathway.png

Educational objective: Sorbitol dehydrogenase is present in the seminal vesicles, ovaries, liver and retina; however, it lacks in retina, shwann cells and kidneys.

Reference: First aid for USMLE step 1, 2013. Page 105.
Educational Objective:
References: ]]

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