WBR0245

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 19-year-old woman is brought to the emergency department (ED) by EMS for transient loss of consciousness. Her roommate 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 experienced a sharp migraine-like headache and vomited twice. She lost consciousness for 10 minutes but regained consciousness by the time EMS arrived. The patient explains that this is not the first time she experiences stroke-like symptoms, and she had multiple similar episodes in the past. Her past medical history is also significant for multiple hospitalizations for tonic-clonic seizures. Upon further investigation, the woman states that she has been intolerant to exercise ever since she was a child. Physical examination in the ED is remarkable for transient weakness in the right upper and lower limbs and a wide-based, ataxic gait. Laboratory studies demonstrate elevated serum lactate concentration. What is the most appropriate diagnostic modality for this patient's condition?]]
Answer A	AnswerA::Brain MRI
Answer A Explanation	AnswerAExp::A patient with stroke-like symptoms and lactic acidosis suggests MELAS syndrome. An MRI might show increased signals in the posterior cerebrum during an episode, but these findings are not usually diagnostic of MELAS.
Answer B	AnswerB::Blood smear
Answer B Explanation	[[AnswerBExp::Hematologic diseases, such as G6PD deficiency, can cause lactic acidosis due to hemolysis. However, lactic acidosis of G6PD is not typically as severe as the acidosis in this patient. Furthermore, G6PD is an X-linked condition and is therefore highly unlikely to occur in a woman.]]
Answer C	AnswerC::Muscle biopsy
Answer C Explanation	AnswerCExp::Ragged red fibers on muscle biopsy are characteristic of mitochondrial diseases, including MELAS.
Answer D	AnswerD::Glucose tolerance test
Answer D Explanation	AnswerDExp::A glucose tolerance test is helpful to diagnose diabetes mellitus.
Answer E	AnswerE::Liver enzymes
Answer E Explanation	AnswerEExp::Liver enzymes are not classically elevated in MELAS.
Right Answer	RightAnswer::C
Explanation	[[Explanation::MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is a hereditary multisystem disorder caused by a mutation of the mitochondrial DNA (mtDNA). The most common mutation in MELAS is the MT-TL1 gene present in the mtDNA that encodes tRNA leucine. Patients with MELAS usually have a normal early development during infancy, and symptoms often start to appear between the age of 2 to 10 years. Individuals with MELAS often develop severe lactic acidosis due to insufficient mitochondrial metabolism activity. Manifestations include generalize tonic-clonic seizures and exercise intolerance (early symptoms), migraine-like headaches, anorexia, and vomiting. Patients usually report recurrent stroke-like episodes before the age of 40. These episodes are associated with altered consciousness, transient hemiparesis, or blindness. Symptoms may be triggered by acute stresses, such as infections. MELAS is usually suspected among patients with symptoms consistent with the disease and lactic acidosis, as suggested by elevated serum lactate levels. The diagnosis is confirmed by muscle biopsy that demonstrates ragged red fibers on trichrome stain. Other helpful tests may include CSF analysis that demonstrates elevated protein and lactate, ECG that may demonstrate evidence of cardiomyopathy, pre-excitation, of heart block, and brain imaging that may demonstrate increased signals in the posterior cerebrum (but not following a major arterial distribution) during a stroke-like episode. Educational Objective: MELAS and other mitochondrial diseases are diagnosed by muscle biopsy, which demonstrates ragged red fibers on trichrome stain. References: DiMauro S, Hirano M. MELAS. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 2001. First Aid 2014 page 86]]
Approved	Approved::Yes
Keyword	WBRKeyword::Mitochondria, WBRKeyword::Mitochonrdial disease, WBRKeyword::MELAS, WBRKeyword::MELAS, WBRKeyword::Ragged red fibers, WBRKeyword::Muscle biopsy
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