View source for WBR1108

{{WBRQuestion
|QuestionAuthor=William J Gibson (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=Vascular, General Principles
|Prompt=A 65-year-old woman is brought to the emergency department by her husband for loss of consciousness during sexual intercourse.  After full cardiovascular and neurological workup she is discharged.  Three days later, she presents for a second episode of loss of consciousness during intercourse.  Further questioning reveals that the husband applied his prescribed anti-anginal agent on his penis to help him achieve an erection prior to intercourse.  The vasodilatory agent responsible for this woman’s presentation is normally produced from which of the following amino acids?
|Explanation=The patient in this vignette has fallen unconscious due to severe hypotension from nitrate overdose.  Isosorbide dinitrate is a commonly prescribed drug used to treat angina pectoris.  It is most commonly administered sublingually.  Patients experiencing angina when doing certain physical activities can often prevent symptoms by taking nitroglycerin 5 to 10 minutes before the activity. Isosorbine dinitrate is converted to nitric oxide by mitochondrial aldehyde dehydrogenase.  Nitric oxide is a potent vasodilator that decreases vascular tone by increasing intracellular cGMP.  Nitric oxide preferentially dilates veins compared to arterioles, thereby decreasing venous system pressure and cardiovascular preload.   Because both end diastolic volume and blood pressure are decreased, myocardial oxygen consumption is decreased and angina is attenuated.

Nitric oxide, known as the 'endothelium-derived relaxing factor', or 'EDRF', is biosynthesized endogenously from L-arginine, oxygen, and NADPH by various nitric oxide synthase (NOS) enzymes.
|AnswerA=Arginine
|AnswerAExp=Endothelial nitric oxide synthase (ENOS) produces the vasodilator nitric oxide from L-Arginine.
|AnswerB=Isoleucine
|AnswerBExp=Isoleucine is one of the amino acids which accumulates to high levels in maple syrup urine disease (Isoleucine, Leucine, Valine – I Love Vermont Maple Syrup).
|AnswerC=Tryptophan
|AnswerCExp=Tryptophan is a precursor for the neurotransmitter serotonin.
|AnswerD=Tyrosine
|AnswerDExp=Tyrosine is used for the synthesis of dopamine.
|AnswerE=Valine
|AnswerEExp=Valine is one of the amino acids which accumulates to high levels in maple syrup urine disease (Isoleucine, Leucine, Valine – I Love Vermont Maple Syrup).
|EducationalObjectives=Endothelial nitric oxide synthase (ENOS) produces the vasodilator nitric oxide from L-Arginine.
|References=First Aid 2015 page 106<br>
First Aid 2014 page 299
|RightAnswer=A
|WBRKeyword=Amino acid, Nitrite, Nitric oxide, Hypotension, Blood pressure, Endothelial, Vascular, Arterial, Angina
|Approved=Yes
}}