View source for WBR0973

{{WBRQuestion
|QuestionAuthor={{Rim}}
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|Prompt=Variation of blood and urine levels of a molecule X is analyzed in two groups of rats fed with either high protein or low protein diet.  Shown below is the structure of the molecule X. 

[[File:Urea.png]]

In this molecule, carbon dioxide is the donor of the carbon and ammonia is the donor of one of the nitrogen.  Which of the following amino acids is the source of the second nitrogen in this molecule?

|Explanation=The molecule depicted in the question is [[urea]].  Urea is formed by two [[nitrogen]], one [[carbon]] and one [[oxygen]] atoms. The carbon and oxygen atoms are derived from carbon dioxide.  The nitrogen atoms are derives from [[ammonia]] and the [[amino acid]] [[aspartate]].

Educational objective: 
The nitrogen atoms are derives from [[ammonia]] and the [[amino acid]] [[aspartate]].

Reference:
First Aid for the USMLE step 1. Page 106.
|AnswerA=Aspartate

|AnswerAExp=[[Aspartate]] is the donor of the second [[nitrogen]] atom of the urea molecule.
|AnswerB=Arginine
|AnswerBExp=[[Arginine]] is not the donor of the second [[nitrogen]] atom of the urea molecule.
|AnswerC=Lysine
|AnswerCExp=[[Lysine]] is not the donor of the second [[nitrogen]] atom of the urea molecule.
|AnswerD=Fumarate
|AnswerDExp=[[Fumarate]] is not the donor of the second [[nitrogen]] atom of the urea molecule.
|AnswerE=Alanine
|AnswerEExp=[[Alanine]] is not the donor of the second [[nitrogen]] atom of the urea molecule.
|RightAnswer=A
|WBRKeyword=urea, aspartate
|Approved=No
}}