WBR0887

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 28-year-old woman recently found to have a positive PPD was started on INH prophylaxis. Six months later, the patient returns to the infectious disease clinic with a small wound on the sole of her foot. She claims she does not remember how she got the wound, and explains that she feels like she has been losing feeling in her feet. She reports having difficulty walking and often losing her shoes without even noticing a difference. She also reports feeling tired and extremely irritable recently. Serum levels of which of the following would you expect to be decreased in this patient?]]
Answer A	AnswerA::Methionine
Answer A Explanation	AnswerAExp::Methionine levels increase with vitamin B6 deficiency.
Answer B	AnswerB::Homocysteine
Answer B Explanation	AnswerBExp::Homocysteine levels increase with vitamin B6 deficiency.
Answer C	AnswerC::Cysteine
Answer C Explanation	AnswerCExp::As vitamin B6 is required for cysteine synthesis, cysteine levels decrease with vitamin B6 deficiency.
Answer D	AnswerD::Succinyl-CoA
Answer D Explanation	AnswerDExp::Succinyl-CoA is a substrate used in heme synthesis. With vitamin B6 deficiency heme synthesis decreases, so does consumption of succinyl-CoA
Answer E	AnswerE::Methylmalonyl-CoA
Answer E Explanation	AnswerEExp::Methylmalonyl-CoA levels are usually not affected by B6 levels.
Right Answer	RightAnswer::C
Explanation	[[Explanation::Vitamin B6 or pyridoxine is an essential vitamin required as a co-factor in many metabolic reactions including heme, GABA, and niacin synthesis among others. Typically, vitamin B6 deficiency presents with irritability, hemolysis, sideroblastic anemia, and peripheral neuropathy. In infants, convulsions can also be seen with significant deficiency. Vitamin B6 deficiency can be seen in patients with severe inflammatory diseases, malnutrition, celiac disease, sickle cell disease, and as a side effect of drugs namely isoniazid, D-penicillamine and OCPs. Pyridoxine is an important cofactor for transforming homocysteine into cysteine. With pyridoxine deficiency, cysteine levels decrease and levels of homocysteine and methionine increase. Educational Objective: Vitamin B6 deficiency, often seen in patients on izoniazid therapy, leads to decreased levels of cysteine. References: ]]
Approved	Approved::Yes
Keyword	WBRKeyword::Izoniazid, WBRKeyword::Vitamin B6, WBRKeyword::Pyridoxine, WBRKeyword::Peripheral neuropathy, WBRKeyword::INH, WBRKeyword::Tuberculosis
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