WBR0347
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]] |
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| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Pathophysiology |
| Sub Category | SubCategory::Hematology |
| Prompt | [[Prompt::A 25-year-old man presents to the physician's office with complaints of blistering vesicles on his face and upper extremities. He reports that he previously had similar episodes in the pasts, all of which occurred following exposure to sunlight. The patient also states 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?
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| Answer A | AnswerA::A |
| Answer A Explanation | [[AnswerAExp::"A" corresponds to ALA synthase. ALA synthase activity is deficient in B6 deficiency.]] |
| Answer B | AnswerB::B |
| Answer B Explanation | [[AnswerBExp::"B" corresponds to ALA dehydratase. ALA dehydratase activity is deficient 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 disease (congenital erythropoietic porphyria).]] |
| 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::Porphyria cutanea tarda (PCT) is caused by the deficiency of the enzyme uroporphyrinogen decarboxylase. Patients with PCT typically present with acute skin eruptions of blistering vesicles that are usually located on the face and upper extremities (sun-exposed body areas) but may also be present different parts of the body a few days following sun exposure. Due to increased concentration of uroporphyrinogen in the urine, the urine appears reddish brown under natural light and pinkish under Wood's lamp. Sun avoidance and reduction of iron stores (phlebotomy and chelation) are important measures in the management of patients with PCT. Educational Objective: Porphyria cutanea tarda (PCT) is caused by a deficiency of uroporphyrinogen decarboxylase. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Porphyria cutanea tarda, WBRKeyword::PCT, WBRKeyword::Photosensitivity, WBRKeyword::Uroporphyrinogen decarboxylase, WBRKeyword::Urine discoloration, WBRKeyword::Uroporphyrinogen, WBRKeyword::Decarboxylase, WBRKeyword::Blisters, WBRKeyword::Vesicles, WBRKeyword::Heme synthesis pathway, WBRKeyword::Wood's lamp |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
