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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{Rim}} (Reviewed by  {{YD}})
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
Line 8: Line 8:
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
Line 20: Line 21:
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=A junior biology student is enrolled in a human physiology laboratory class.  One of the assignment involves collecting each student's urine for pH measurement and observation under the microscope for casts, crystals and cells. The student uses a small amount of urine for the assignment and forgets the flask open. When the student is cleaning his bench a couple of hours later, he notices that the urine in the flask turned dark brown. The student gets worried and asks his professor about the cause of the change in color of his urine.  The professor suspects a genetic condition caused by a deficiency of the enzyme involved in the breakdown of tyrosine. Which of the following is a long term complication of this genetic condition?
|Prompt=A biology student collects 50 urine samples for pH measurement and microscopic analysis. The samples are obtained from 21-year-old healthy individuals and stored in sterile cups pending analysis. The student forgets to close one cup and leaves the cup open overnight. The next day, the student returns to his bench and notices that the urine color in the open cup turned bluish/black. The student inquires about his finding and eventually discovers that the sample was collected from a subject with a genetic disorder characterized by a deficiency of an enzyme involved in tyrosine breakdown. Which of the following is a long-term complication of this genetic disorder?
|Explanation=[[Alkaptonuria]] is a rare autosomal recessive condition that is due to a defect in the enzyme homogenistic acid oxidase, which participates in the degradation of [[tyrosine]] to [[fumarate]]. As a result, homogentisic acid and its oxide, called alkapton, accumulate in the blood and are excreted in urine in large amounts. Excessive homogentisic acid causes damage to cartilage leading to [[arthralgia]].  Alkaptonuria is often asymptomatic, but the [[sclera]] of the eyes may be pigmented, and the skin may be darkened in sun exposed areas and around sweat glands; sweat may be coloured brown. Urine may turn brown or even inky black if collected and left exposed to open air, especially when left standing for a period of time.
|Explanation=[[Alkaptonuria]] is an autosomal recessive genetic disorder characterized by deficiency of homogentisate 1,2-dioxygenase enzyme, which is involved in tyrosine breakdown to fumarate. The enzyme normally converts homogentisic acid (HGA) to maleylacetoacetic acid. In alkaptonuria, the blood and urinary concentrations of homogentisic acid (HGA) and its oxide (benzoquinone acetic acid) gradually increase and cause early ochronosis (HGA deposition) by the age of 30. Excessive ochronosis typically manifests with arthritis with lumbar spine calcification, disc flattening, and osteophyte formation. In addition, alkaptonuria results in HGA deposition that occurs in the skin, tendons, urine, eyes, and ears (resulting in bluish/black pigmentation in skin, sweat, urine, cerumen, sclera, conjunctiva, and concha due to HGA oxidation to melanin-like compounds) and valvular calcification or regurgitation, recurrent nephrolithasis, and prostate stones. The diagnosis is often made upon detection of high concentration of urinary HGA. Genetic testing confirms the diagnosis, demonstrating a mutation in the ''HGD'' gene. Management of alkaptonuria is directed towards management of manifestations and includes physical therapy and surgical interventions when needed.
 
'''Educational objective:'''
[[Alkaptonuria]] is a benign autosomal recessive condition that is due to a defect in the enzyme homogenistic acid oxidase.  It can be asymptomatic, but also can be associated with arthralgia, brown pigmenented sclera and darkening of the urine when left standing.
 
'''References:'''
First aid for USMLE step 1, 2013. Page 108.
|AnswerA=Blindness
|AnswerA=Blindness
|AnswerAExp=[[Alkaptonuria]] is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase.  It can be associated with brown pigmenented sclera but not with blindness.
|AnswerAExp=[[Alkaptonuria]] is associated with bluish/black pigmented sclerae. However, it is not associated with blindness.
|AnswerB=Cancer
|AnswerB=Cancer
|AnswerBExp=[[Alkaptonuria]] is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase.  It is not associated with cancer.
|AnswerBExp=[[Alkaptonuria]] is a benign condition that is not associated with the development of cancer.
|AnswerC=Arthralgia
|AnswerC=Arthralgia
|AnswerCExp=[[Alkaptonuria]] is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase.  It is associated with arthralgia due to the excessive homogentisic acid deposition and subsequent damage to the cartilage.
|AnswerCExp=[[Alkaptonuria]] is associated with arthralgia due to the excessive homogentisic acid deposition (ochronosis).
|AnswerD=Hearing loss
|AnswerD=Hearing loss
|AnswerDExp=[[Alkaptonuria]] is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase. It is not associated with darkening of the ear but not hearing loss.
|AnswerDExp=[[Alkaptonuria]] is associated with pigmented ears, concha, and cerumen. However, it is not associated with hearing loss.
|AnswerE=Seizures
|AnswerE=Seizures
|AnswerEExp=[[Alkaptonuria]] is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase. It is not associated with seizures.
|AnswerEExp=[[Alkaptonuria]] is not associated with development of seizures.
|EducationalObjectives=[[Alkaptonuria]] is an autosomal recessive genetic disorder characterized by deficiency of homogentisate 1,2-dioxygenase enzyme, which is involved in tyrosine breakdown and normally converts homogentisic acid (HGA) to maleylacetoacetic acid. Excessive ochronosis (HGA deposition) typically manifests with arthritis with lumbar spine calcification, disc flattening, and osteophyte formation. In addition, alkaptonuria results in HGA deposition that occurs in the skin, tendons, urine, eyes, and ears.
|References=Introne WJ, Gahl WA. Alkaptonuria. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews®. Seattle (WA): University of Washington, Seattle; 2003. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1454/<br>
Menon IA, Persad SD, Haberman HF, et al. Characterization of the pigment from homogentisic acid and urine and tissue from an alkaptonuria patient. Biochem Cell Biol. 1991;69(4):269-73.<br>
First Aid 2014 page 111.
|RightAnswer=C
|RightAnswer=C
|WBRKeyword=Alkaptonuria, arthralgia
|WBRKeyword=Alkaptonuria, Arthralgia, HGA, Complication, Urine, Dark color, Homogentisate 1,2-dioxygenase, Autosomal recessive, Genetic disorder, Ochronosis, Pigment, Pigmentation, Melanin, Tyrosine breakdown, Homogentisic acid, Fumarate, Benzoquinone acetic acid
|Approved=No
|Approved=Yes
}}
}}

Latest revision as of 02:17, 28 October 2020
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::A biology student collects 50 urine samples for pH measurement and microscopic analysis. The samples are obtained from 21-year-old healthy individuals and stored in sterile cups pending analysis. The student forgets to close one cup and leaves the cup open overnight. The next day, the student returns to his bench and notices that the urine color in the open cup turned bluish/black. The student inquires about his finding and eventually discovers that the sample was collected from a subject with a genetic disorder characterized by a deficiency of an enzyme involved in tyrosine breakdown. Which of the following is a long-term complication of this genetic disorder?]]
Answer A AnswerA::Blindness
Answer A Explanation [[AnswerAExp::Alkaptonuria is associated with bluish/black pigmented sclerae. However, it is not associated with blindness.]]
Answer B AnswerB::Cancer
Answer B Explanation [[AnswerBExp::Alkaptonuria is a benign condition that is not associated with the development of cancer.]]
Answer C AnswerC::Arthralgia
Answer C Explanation [[AnswerCExp::Alkaptonuria is associated with arthralgia due to the excessive homogentisic acid deposition (ochronosis).]]
Answer D AnswerD::Hearing loss
Answer D Explanation [[AnswerDExp::Alkaptonuria is associated with pigmented ears, concha, and cerumen. However, it is not associated with hearing loss.]]
Answer E AnswerE::Seizures
Answer E Explanation [[AnswerEExp::Alkaptonuria is not associated with development of seizures.]]
Right Answer RightAnswer::C
Explanation [[Explanation::Alkaptonuria is an autosomal recessive genetic disorder characterized by deficiency of homogentisate 1,2-dioxygenase enzyme, which is involved in tyrosine breakdown to fumarate. The enzyme normally converts homogentisic acid (HGA) to maleylacetoacetic acid. In alkaptonuria, the blood and urinary concentrations of homogentisic acid (HGA) and its oxide (benzoquinone acetic acid) gradually increase and cause early ochronosis (HGA deposition) by the age of 30. Excessive ochronosis typically manifests with arthritis with lumbar spine calcification, disc flattening, and osteophyte formation. In addition, alkaptonuria results in HGA deposition that occurs in the skin, tendons, urine, eyes, and ears (resulting in bluish/black pigmentation in skin, sweat, urine, cerumen, sclera, conjunctiva, and concha due to HGA oxidation to melanin-like compounds) and valvular calcification or regurgitation, recurrent nephrolithasis, and prostate stones. The diagnosis is often made upon detection of high concentration of urinary HGA. Genetic testing confirms the diagnosis, demonstrating a mutation in the HGD gene. Management of alkaptonuria is directed towards management of manifestations and includes physical therapy and surgical interventions when needed.

Educational Objective: Alkaptonuria is an autosomal recessive genetic disorder characterized by deficiency of homogentisate 1,2-dioxygenase enzyme, which is involved in tyrosine breakdown and normally converts homogentisic acid (HGA) to maleylacetoacetic acid. Excessive ochronosis (HGA deposition) typically manifests with arthritis with lumbar spine calcification, disc flattening, and osteophyte formation. In addition, alkaptonuria results in HGA deposition that occurs in the skin, tendons, urine, eyes, and ears.
References: Introne WJ, Gahl WA. Alkaptonuria. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews®. Seattle (WA): University of Washington, Seattle; 2003. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1454/
Menon IA, Persad SD, Haberman HF, et al. Characterization of the pigment from homogentisic acid and urine and tissue from an alkaptonuria patient. Biochem Cell Biol. 1991;69(4):269-73.
First Aid 2014 page 111.]]

Approved Approved::Yes
Keyword WBRKeyword::Alkaptonuria, WBRKeyword::Arthralgia, WBRKeyword::HGA, WBRKeyword::Complication, WBRKeyword::Urine, WBRKeyword::Dark color, WBRKeyword::Homogentisate 1, WBRKeyword::2-dioxygenase, WBRKeyword::Autosomal recessive, WBRKeyword::Genetic disorder, WBRKeyword::Ochronosis, WBRKeyword::Pigment, WBRKeyword::Pigmentation, WBRKeyword::Melanin, WBRKeyword::Tyrosine breakdown, WBRKeyword::Homogentisic acid, WBRKeyword::Fumarate, WBRKeyword::Benzoquinone acetic acid
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