WBR0243

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::Neurology, SubCategory::General Principles
Prompt	[[Prompt::An 11 year old boy is brought to his pediatrician for recurrent episodes of involuntary muscle twitching. He reports that these episodes have grown more frequent and increased in duration over the past year. The patient also suffers from poor vision at night, and exercise intolerance that began at the age of six. A maternal aunt has epilepsy. The patient has achieved all developmental milestones, but has especially short stature relative to his peers. Which of the following is true of this patient’s condition?]]
Answer A	AnswerA::Associated with elevated bilirubin
Answer A Explanation	AnswerAExp::'''Incorrect''' - MERRF is associated with elevated lactic acid not associated with hemolysis. Therefore, bilirubin is not elevated in MERRF.
Answer B	AnswerB::Associated with ketoacidosis
Answer B Explanation	AnswerBExp::'''Incorrect''' - MERFF is not associated with ketoacidosis. MERRF can cause lactic acidosis. However a related mitochondrial condition, MELAS, is more associated with lactic acidosis.
Answer C	AnswerC::Associated with respiratory acidosis
Answer C Explanation	AnswerCExp::'''Incorrect''' - MERRF is not associated with respiratory acidosis. MERRF can cause lactic acidosis. However a related mitochondrial condition, MELAS, is more associated with lactic acidosis.
Answer D	AnswerD::Variable expressivity due to heteroplasmy
Answer D Explanation	[[AnswerDExp::Correct - Expressivity refers to the range of symptoms a patient shows. Heteroplasmy is the presence of a mixture of more than one type of an mitochondrial genome within a cell or individual. Some individuals and even particular tissues within certain individuals will differ in the proportion of mitochondria carrying a mutant allele. Thus, the variable phenotype of MERRF can be caused by differing proportions of mutant mitochondria.]]
Answer E	AnswerE::Variable expressivity due to X-inactivation in females
Answer E Explanation	AnswerEExp::'''Incorrect''' - MERRF is not an X-linked disease. Variable expressivity due to X-inactivation in females is true of fragile X syndrome.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The patient in this vignette is suffering from myoclonic epilepsy with ragged red fibers (MERRF). It involves the following characteristics: progressive myoclonic epilepsy, short stature, hearing loss, lactic acidosis, exercise intolerance, and poor night vision. Muscle biopsy of affected patients will demonstrate clumps of diseased mitochondria and appear as ragged red fibers on trichome stain. Because MERRF is a mitochondrial condition, it can only be inherited through the mother. The severity of the patient’s symptoms depends on the proportion of mutated mitochondria the fertilized egg contains. Expressivity refers to the range of symptoms a patient shows. Heteroplasmy is the presence of a mixture of more than one type of an mitochondrial genome within a cell or individual. Some individuals and even particular tissues within certain individuals will differ in the proportion of mitochondria carrying a mutant allele. Thus, the variable phenotype of MERRF can be caused by differing proportions of mutant mitochondria. Educational Objective: The variable phenotype of MERRF can be caused by differing proportions of mutant mitochondria in different patient’s tissues. References: First Aid 2012 page 89. Educational Objective: References: ]]
Approved	Approved::Yes
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