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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor={{Rim}} {{Alison}}
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Pathophysiology
|MainCategory=Pathophysiology
Line 20: Line 20:
|MainCategory=Pathophysiology
|MainCategory=Pathophysiology
|SubCategory=Hematology
|SubCategory=Hematology
|Prompt=A 25 year old male patient, previously healthy, presents to the physician's office complaining of blistering vesicles over different parts of his body. He reports that he has had previous episodes in the past, all of which occur following exposure to sunlight. The patient then adds that he also notices that his urine has a reddish brown discoloration under natural light. Based on the simplified diagram of heme synthesis shown below, which of the following steps is most likely inhibited in this patient?
|Prompt=A 25-year-old male presents to the physician's office with complaints of blistering vesicles on various parts of his body. He reports that he had previous episodes, all of which occured following exposure to sunlight, and that his urine has a reddish brown discoloration under natural light. Based on the simplified diagram of heme synthesis illustrated below, which of the following steps is most likely inhibited in this patient?


[[Image:WBR0344.png|500px]]
[[Image:WBR0344.png|500px]]
|Explanation=The patient is most likely presenting with porphyria cutanea tarda (PCT). Porphyria cutanea tarda is characterized by the deficiency of the enzyme uroporphyrinogen decarboxylase. In PCT, patients typically have acute skin eruptions of blistering vesicles that occupy different parts of the patient's body. These vesicles appear in sun-exposed areas and are exacerbated by sunlight and typically arise a few days after exposure to sun. Due to increased levels of uroporphyrinogen in the urine, the color of the urine becomes reddish brown under natural light and pinkish under Wood's lamp. PCT may be familial or sporadic. Sun avoidance, and reduction of iron stores (phlebotomy and chelation) are important in the management of the patient.
|Explanation=The patient in this scenario most likely has [[porphyria cutanea tarda]] (PCT). [[Porphyria cutanea tarda]], which may be familial or sporadic, is characterized by the deficiency of the enzyme uroporphyrinogen decarboxylase. In PCT, patients typically present with acute skin eruptions of blistering vesicles, which occupy different parts of the body. Typically appearing a few days following sun exposure, these vesicles are exacerbated by sunlight. Due to increased levels of uroporphyrinogen in the urine, the urine becomes reddish brown under natural light and pinkish under Wood's lamp. Sun avoidance, and reduction of iron stores (phlebotomy and chelation) are important in the management of the patient.


(To note, the image above is simplified. Some intermediate products and enzymes are not listed in this illustration of heme synthesis).
(To note, the image above is simplified. Some intermediate products and enzymes are not listed in this illustration of heme synthesis).


Educational Objective:
|EducationalObjectives= [[Porphyria cutanea tarda]] is characterized by a deficiency of uroporphyrinogen decarboxylase.
Porphyria cutanea tarda is characterized by a deficiency of uroporphyrinogen decarboxylase.
 
|References= Munoz-Santos C, Guilabert A, Monero N, et al. Familial and sporadic porphyria cutanea tarda: clinical and biochemical features and risk factors in 152 patients. Medicine (Baltimore). 2010;89(2):69-74.


Reference:
Munoz-Santos C, Guilabert A, Monero N, et al. Familial and sporadic porphyria cutanea tarda: clinical and biochemical features and risk factors in 152 patients. Medicine (Baltimore). 2010;89(2):69-74.
|AnswerA=A
|AnswerA=A
|AnswerAExp="A" corresponds to ALA synthase, whose activity is decreased in B6 deficiency.
|AnswerAExp="A" corresponds to ALA synthase. ALA synthase activity is decreased in [[B6 deficiency]].
|AnswerB=B
|AnswerB=B
|AnswerBExp="B" corresponds to ALA dehydratase, whose activity is decreased in lead toxicity.
|AnswerBExp="B" corresponds to ALA dehydratase. ALA dehydratase activity is decreased in [[lead toxicity]].
|AnswerC=C
|AnswerC=C
|AnswerCExp="C" corresponds to porphobilinogen deaminase, deficient in acute intermittent porphyria.
|AnswerCExp="C" corresponds to porphobilinogen deaminase, deficient in acute intermittent [[porphyria]].
|AnswerD=D
|AnswerD=D
|AnswerDExp="D" corresponds to uroporphyrinogen III synthase, which is not clinically relevant.
|AnswerDExp="D" corresponds to uroporphyrinogen III synthase. Uroporphyrinogen III synthase deficiency is associated with [[Gunther's disease]].  
|AnswerE=E
|AnswerE=E
|AnswerEExp="E" corresponds to uroporphyrinogen decarboxylase, deficient in porphyria cutanea tarda (PCT).
|AnswerEExp="E" corresponds to uroporphyrinogen decarboxylase, deficient in [[Porphyria cutanea tarda]] (PCT).
|RightAnswer=E
|RightAnswer=E
|WBRKeyword=porphyria, cutanea, tarda, PCT, photosensitivity, urine, pink, pinkish, red, reddish, brown, brownish, color, discoloration, uroporphyrinogen, decarboxylase, blister, vesicle, blisters, vesicles, heme, pathway, Wood, Wood's, lamp, ultraviolet
|WBRKeyword=porphyria, cutanea, tarda, PCT, photosensitivity, urine discoloration, uroporphyrinogen, decarboxylase, blisters, vesicles, heme pathway, Wood's lamp, ultraviolet
|Approved=No
|Approved=Yes
}}
}}

Revision as of 15:46, 16 July 2014
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Pathophysiology
Sub Category SubCategory::Hematology
Prompt [[Prompt::A 25-year-old male presents to the physician's office with complaints of blistering vesicles on various parts of his body. He reports that he had previous episodes, all of which occured following exposure to sunlight, and that his urine has a reddish brown discoloration under natural light. Based on the simplified diagram of heme synthesis illustrated below, which of the following steps is most likely inhibited in this patient?

]]

Answer A AnswerA::A
Answer A Explanation [[AnswerAExp::"A" corresponds to ALA synthase. ALA synthase activity is decreased in B6 deficiency.]]
Answer B AnswerB::B
Answer B Explanation [[AnswerBExp::"B" corresponds to ALA dehydratase. ALA dehydratase activity is decreased in lead toxicity.]]
Answer C AnswerC::C
Answer C Explanation [[AnswerCExp::"C" corresponds to porphobilinogen deaminase, deficient in acute intermittent porphyria.]]
Answer D AnswerD::D
Answer D Explanation [[AnswerDExp::"D" corresponds to uroporphyrinogen III synthase. Uroporphyrinogen III synthase deficiency is associated with Gunther's disease.]]
Answer E AnswerE::E
Answer E Explanation [[AnswerEExp::"E" corresponds to uroporphyrinogen decarboxylase, deficient in Porphyria cutanea tarda (PCT).]]
Right Answer RightAnswer::E
Explanation [[Explanation::The patient in this scenario most likely has porphyria cutanea tarda (PCT). Porphyria cutanea tarda, which may be familial or sporadic, is characterized by the deficiency of the enzyme uroporphyrinogen decarboxylase. In PCT, patients typically present with acute skin eruptions of blistering vesicles, which occupy different parts of the body. Typically appearing a few days following sun exposure, these vesicles are exacerbated by sunlight. Due to increased levels of uroporphyrinogen in the urine, the urine becomes reddish brown under natural light and pinkish under Wood's lamp. Sun avoidance, and reduction of iron stores (phlebotomy and chelation) are important in the management of the patient.

(To note, the image above is simplified. Some intermediate products and enzymes are not listed in this illustration of heme synthesis).
Educational Objective: Porphyria cutanea tarda is characterized by a deficiency of uroporphyrinogen decarboxylase.
References: Munoz-Santos C, Guilabert A, Monero N, et al. Familial and sporadic porphyria cutanea tarda: clinical and biochemical features and risk factors in 152 patients. Medicine (Baltimore). 2010;89(2):69-74.]]

Approved Approved::Yes
Keyword WBRKeyword::porphyria, WBRKeyword::cutanea, WBRKeyword::tarda, WBRKeyword::PCT, WBRKeyword::photosensitivity, WBRKeyword::urine discoloration, WBRKeyword::uroporphyrinogen, WBRKeyword::decarboxylase, WBRKeyword::blisters, WBRKeyword::vesicles, WBRKeyword::heme pathway, WBRKeyword::Wood's lamp, WBRKeyword::ultraviolet
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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