WBR0244

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::Head and Neck, SubCategory::Neurology, SubCategory::General Principles
Prompt	[[Prompt::A 16 year old boy is referred to an optometrist for poor vision in his right eye 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 cecocentral scotoma. The child is issued corrective lenses for presumed amblyopia. Three weeks later, the patient returns complaining that his corrective lenses no longer allow him to see properly. Reassessment of visual acuity demonstrates 20/80 vision in the left eye and near total blindness of the right eye. Fundoscopic examination reveals tortuous and dilated retinal blood vessels, with a crowded and swollen nerve fiber layer. MRI is negative for a cerebral mass. The physician informs the patient that he will become completely blind over the next several months. The mode of inheritance of the most likely condition is also shared by which of the following conditions?]]
Answer A	AnswerA::Glucose-6-Phosphate Dehydrogenase Deficiency
Answer A Explanation	AnswerAExp::'''Incorrect''' - G6PD is an X-linked recessive disease. This patient is suffering from Leber’s Hereditary Optic Neuropathy, a mitochondrial disease.
Answer B	AnswerB::Phenylketonuria
Answer B Explanation	AnswerBExp::'''Incorrect''' - Phenylketonuria is an autosomal recessive disease. This patient is suffering from Leber’s Hereditary Optic Neuropathy, a mitochondrial disease.
Answer C	AnswerC::Hypophosphatemic Rickets
Answer C Explanation	AnswerCExp::'''Incorrect'''- Hypophosphatemic rickets is an X-linked dominant disorder. This patient is suffering from Leber’s Hereditary Optic Neuropathy, a mitochondrial disease.
Answer D	AnswerD::Multiple Endocrine Neoplasia
Answer D Explanation	AnswerDExp::'''Incorrect''' - The multiple endocrine neoplasia syndromes (Type 1,2A and 2B) are autosomal dominant diseases. This patient is suffering from Leber’s Hereditary Optic Neuropathy, a mitochondrial disease.
Answer E	AnswerE::Myoclonic epilepsy with ragged red fibers
Answer E Explanation	AnswerEExp::'''Correct''' - Myoclonic epilepsy with ragged red fibers is a disease with a mitochondrial pattern of inheritance.
Right Answer	RightAnswer::E
Explanation	[[Explanation::The patient in this vignette is suffering from Leber’s hereditary optic neuropathy (LHON). LHON is a mitochondrially inherited degeneration of retinal ganglion cells that leads to an acute or subacute loss of central vision; this affects predominantly young adult males. Clinically, there is an acute onset of visual loss, first in one eye, and then a few weeks to months later in the other. Onset is usually young adulthood, but age range at onset from 7-75 is reported. Loss of visual acuity typically evolves to very severe optic atrophy and permanent decrease of visual acuity. LHON is caused by mutations in mitochondrial NADH dehydrogenase. The disorder can only be inherited maternally because only the mother contributes mitochondria 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. References: First Aid 2012 page 89. Educational Objective: References: ]]
Approved	Approved::Yes
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