WBR0597

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathophysiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 74-year-old man presents to the physician's office for a clouded dim vision that has been progressively worsening over the past 6 months. The patient further explains that he has difficulty seeing objects at night and sees normal lights too bright. His past medical history is significant for uncontrolled type 2 diabetes mellitus on daily insulin injections, hypertension on metoprolol and lisinopril, and dyslipidemia on atorvastatin. When asked, the physician explains to the patient the etiology of the patient's symptoms. Which of the following is true regarding the pathophysiology of the patient's condition?]]
Answer A	AnswerA::Optic disc swelling that results from an elevation in intracranial pressure
Answer A Explanation	[[AnswerAExp::Papilledema is characterized by optic disc swelling that results from an elevation in intracranial pressure. Patients with elevated intracranial pressures may remain asymptomatic or may present with visual and extravisual symptoms. Visual symptoms of papilledema include transient visual flickering, blurring of vision, constricted visual field, diplopia, and decreased perception of color.]]
Answer B	AnswerB::Absence of a key transfer enzyme and subsequent shunting of the pathway towards the formation of galactitol
Answer B Explanation	[[AnswerBExp::Classic galactosemia is an autosomal recessive disorder characterized by the absence of galactose-1-phosphate uridyltransferase enzyme and subsequent shunting of the galactose metabolism pathway towards the formation of galactitol. Classic galactosemia is associated with development of infantile cataracts, which is unusual among elderly patients.]]
Answer C	AnswerC::Accumulation of glucose by-products in organs and structures that lack enzymes which normally allow oxidation to fructose
Answer C Explanation	[[AnswerCExp::Cataract is caused by sorbitol accumulation in diabetic patients. The eye retina, the kidney, and schwann cells of the nervous system are especially susceptible to sorbitol accumulation because they normally lack sorbitol dehydrogenase that depletes sorbitol via the formation of fructose (an oxidation reaction).]]
Answer D	AnswerD::A bilateral lesion at the level of the Meyer's loop in the cerebral cortex
Answer D Explanation	[[AnswerDExp::Meyer's loop is located in the temporal lobe and is a special region of the visual pathway. If lesioned, patients often complain of superior homonymous quadrantanopia. In the event of bilateral lesions, the expected visual loss would be vision loss of the superior half of the visual field.]]
Answer E	AnswerE::Degeneration of the central region of the retina due to accumulation of deposited material in the extracellular region beneath the retinal pigmented epithelium
Answer E Explanation	[[AnswerEExp::Macular degeneration is the degeneration of the central region within the retina due to extracellular deposition of material beneath the retinal pigmented epithelium. Age-related macular denegeration (ARMD) is the leading cause of blindness in America among patients above the age of 60 years. The disease manifests with an increase in the number of floaters, flashes of light on peripheral vision, and a positive Amsler grid test. Amsler grid is a grid that uses horizontal and vertical lines and a small dot in the center. The patient looks at the grid using each eye separately. A positive (abnormal) test is when the patient sees the straight lines as curved. Positive Amsler test usually signifies the presence of a retinal disease, such as ARMD.]]
Right Answer	RightAnswer::C
Explanation	[[Explanation::Cataracts is a frequent cause of vision loss among the elderly and a common complication of diabetes mellitus. It results from the accumulation of sorbitol, which is normally produced from glucose by the action of aldose reductase. The retina, the kidney, and schwann cells of the nervous system are especially susceptible to sorbitol accumulation because they normally lack sorbitol dehydrogenase that depletes sorbitol by an oxidation reaction via the formation of fructose. Cataract is characterized by the opacification of the lens. It is a painless condition and often occurs bilaterally. Classical symptoms are progressive visual loss, clouding of vision, and glare when looking at lights that appear abnormally bright. Educational Objective: Cataract is caused by sorbitol accumulation in diabetic patients. The eye retina, the kidney, and schwann cells of the nervous system are especially susceptible to sorbitol accumulation because they normally lack sorbitol dehydrogenase that depletes sorbitol via the formation of fructose (an oxidation reaction). References: Omotosho IO, Obisesan OB, Oluleye O. Sorbitol dehydrogenase activity in diabetes mellitus and cataract patients. J Applied Med Sci. 2014;3(1):61-5. Faes L, Bodmer NS, Bachmann LM, et al. Diagnostic accuracy of the Amsler grid and the preferential hyperacuity perimetry in the screening of patients with age-related macular degeneration: systematic review and meta-analysis. Eye (Lond). 2014;28(7):788-96. First Aid 2014 page 107]]
Approved	Approved::Yes
Keyword	WBRKeyword::Cataract, WBRKeyword::Sorbitol, WBRKeyword::Accumulation, WBRKeyword::Oxidation, WBRKeyword::Diabetes mellitus, WBRKeyword::Complication, WBRKeyword::Fructose, WBRKeyword::Sorbitol dehydrogenase, WBRKeyword::Retina
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