WBR0347

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathophysiology
Sub Category	SubCategory::Hematology
Prompt	[[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? ]]
Answer A	AnswerA::A
Answer A Explanation	AnswerAExp::"A" corresponds to ALA synthase, whose activity is decreased in B6 deficiency.
Answer B	AnswerB::B
Answer B Explanation	AnswerBExp::"B" corresponds to ALA dehydratase, whose 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, which is not clinically relevant.
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 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. (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. 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. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::porphyria, WBRKeyword::cutanea, WBRKeyword::tarda, WBRKeyword::PCT, WBRKeyword::photosensitivity, WBRKeyword::urine, WBRKeyword::pink, WBRKeyword::pinkish, WBRKeyword::red, WBRKeyword::reddish, WBRKeyword::brown, WBRKeyword::brownish, WBRKeyword::color, WBRKeyword::discoloration, WBRKeyword::uroporphyrinogen, WBRKeyword::decarboxylase, WBRKeyword::blister, WBRKeyword::vesicle, WBRKeyword::blisters, WBRKeyword::vesicles, WBRKeyword::heme, WBRKeyword::pathway, WBRKeyword::Wood, WBRKeyword::Wood's, WBRKeyword::lamp, WBRKeyword::ultraviolet
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