WBR0987
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Biochemistry |
| Sub Category | SubCategory::General Principles |
| Prompt | [[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?]] |
| Answer A | AnswerA::Blindness |
| Answer A Explanation | [[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.]] |
| Answer B | AnswerB::Cancer |
| Answer B Explanation | [[AnswerBExp::Alkaptonuria is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase. It is not associated with cancer.]] |
| Answer C | AnswerC::Arthralgia |
| Answer C Explanation | [[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.]] |
| Answer D | AnswerD::Hearing loss |
| Answer D Explanation | [[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.]] |
| Answer E | AnswerE::Seizures |
| Answer E Explanation | [[AnswerEExp::Alkaptonuria is a benign condition that is due to a defect in the enzyme homogenistic acid oxidase. It is not associated with seizures.]] |
| Right Answer | RightAnswer::C |
| Explanation | [[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.
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. |
| Approved | Approved::No |
| Keyword | WBRKeyword::Alkaptonuria, WBRKeyword::arthralgia |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |