WBR0245

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics, MainCategory::Pathophysiology
Sub Category	SubCategory::Neurology, SubCategory::General Principles
Prompt	[[Prompt::A 36 year old woman is brought by her husband to the emergency room after she suffered a seizure. She reports that she was watching television when she began to see the colors on the screen blend and the room began to “wave”. As her vision normalized she began to experience a sharp migraine headache, became nauseated and vomited. She lost consciousness for 10 minutes but regained consciousness by the time the paramedics arrived. The patient has normal speech, but marked weakness of the right upper and lower limbs. Her gait is wide-based and ataxic. Laboratory studies are significant for a serum lactate concentration of 58.4 mmol/L (reference <2.1 mmol/L). Which of the following tests would be most specific for diagnosing the condition which would best explain the patient’s symptoms?]]
Answer A	AnswerA::MRI
Answer A Explanation	[[AnswerAExp::Incorrect - A patient with stroke-like symptoms and lactic acidosis suggests MELAS syndrome. An MRI might show ventricular enlargement but an MRI would be nonspecific for MELAS. Many other conditions including stroke could cause macroscopic alterations in the brain that could be visualized by MRI.]]
Answer B	AnswerB::Blood smear
Answer B Explanation	[[AnswerBExp::Incorrect - Certain conditions such as G6PD can cause lactic acidosis due to widespread hemolysis. However, the lactic acidosis of G6PD is not typically as severe as the acidosis in this patient. Furthermore, G6PD is an X-linked condition and therefore highly unlikely to occur in a female. Blood smear would show bite cells.]]
Answer C	AnswerC::Muscle biopsy
Answer C Explanation	AnswerCExp::'''Correct''' - Ragged red fibers on muscle biopsy is specific for mitochondrial diseases, including MELAS.
Answer D	AnswerD::Glucose tolerance test
Answer D Explanation	AnswerDExp::'''Incorrect''' - A glucose tolerance could be used to diagnose diabetes.
Answer E	AnswerE::Liver enzymes
Answer E Explanation	AnswerEExp::'''Incorrect''' - Liver enzymes (ALT, AST) can indicate hepatitis which is not a feature of MELAS.
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette is suffering from mitochondrial myopathy, encephalomyopathy, lactic acidosis, stroke-like symptoms (MELAS Syndrome). MELAS is a hereditary disorder affecting the mitochondria. Individuals with MELAS develop severe lactic acidosis due to insufficient activity mitochondrial metabolism. Therefore, the body symptoms most affected by MELAS are those most dependent on oxidative phosphorylation (ie the nervous system and muscles). Early symptoms may include muscle weakness and pain, recurrent headaches, loss of appetite, vomiting, and seizures. Most affected individuals experience stroke-like episodes beginning before age 40. These episodes often involve temporary muscle weakness on one side of the body (hemiparesis), altered consciousness, vision abnormalities, seizures, and severe headaches resembling migraines. Repeated stroke-like episodes can progressively damage the brain, leading to vision loss, problems with movement, and a loss of intellectual function (dementia). Muscle biopsy of affected patients will demonstrate clumps of diseased mitochondria and appear as ragged red fibers on trichome stain. Educational Objective: MELAS and other mitochondrial diseases cause ragged red fibers on muscle biopsy. References: First Aid 2012 page 89. Educational Objective: References: ]]
Approved	Approved::Yes
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