WBR0244
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| Author | [[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Genetics |
| Sub Category | General Principles |
| Prompt | A 19-year-old man is referred for poor vision following annual school vision testing. Visual acuity testing reveals 20/30 vision in the left eye and 20/400 vision in the right eye with a centrocecal scotoma, and the patient was prescribed corrective lenses. Three weeks later, the patient returns complaining that his corrective lenses no longer allow him to see properly. Reassessment of his visual acuity demonstrates 20/80 vision in the left eye and near total blindness in the right eye. Fundoscopic examination reveals tortuous and dilated retinal blood vessels with a crowded and swollen nerve fiber layer. Head MRI is negative for an intracranial mass. The physician informs the patient that he will most likely become completely blind over the next several months. Which disease also shares the same mode of inheritance as the patient's condition? |
| Answer A | Glucose-6-Phosphate dehydrogenase deficiency |
| Answer A Explanation | Glucose-6-Phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive genetic disease. |
| Answer B | Phenylketonuria |
| Answer B Explanation | Phenylketonuria is an autosomal recessive disease. |
| Answer C | Hypophosphatemic rickets |
| Answer C Explanation | Hypophosphatemic rickets is an X-linked dominant disorder. |
| Answer D | Multiple endocrine neoplasia |
| Answer D Explanation | Mltiple endocrine neoplasia (MEN) syndromes (Types 1, 2A, and 2B) are autosomal dominant diseases. |
| Answer E | Myoclonic epilepsy with ragged red fibers |
| Answer E Explanation | Myoclonic epilepsy with ragged red fibers is a disease with a mitochondrial pattern of inheritance. |
| Right Answer | E |
| Explanation | [[Explanation::Leber’s hereditary optic neuropathy (LHON) is a genetic disease caused by a mutations in the mitochondrial DNA in genes that express NADH dehydrogenase. It is characterized by bilateral, painless, subacute vision loss that most commonly affects young male adults. Initially, patients often complain of a central scotoma in one eye followed by another scotoma in the other eye approximately 2-3 months later. Loss of visual acuity typically evolves to very severe optic atrophy and permanent decrease of visual acuity. The disorder can only be inherited maternally because only the mother's mitochondria are transmitted to the developing embryo. This mitochondrial inheritance is also true of myoclonic epilepsy with ragged red fibers (MERRF). Educational Objective: Leber’s hereditary optic neuropathy and myoclonic epilepsy with ragged red fibers are both mitochondrially inherited disorders. |
| Approved | Yes |
| Keyword | Leber's hereditary optic neuropathy, LHON, Mitochondrial, Mitochondria, Vision, Blindness, MERRF, Mode of inheritance |
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