WBR0977
| 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 is absent in retina, schwann 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.
Educational objective: Sorbitol dehydrogenase is present in the seminal vesicles, ovaries, liver and retina; however, it is absent in the retina, schwann cells and kidneys. Reference:
First aid for USMLE step 1, 2013. Page 105. |
| Approved | Approved::No |
| Keyword | |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
