USMLE Forums banner
1 - 10 of 10 Posts

·
Registered
Joined
·
590 Posts
Discussion Starter · #1 ·
A 58-year old woman presents with a chief complaint of skin fragility. There is diffuse blistering and scarring on her hands and distal forearms bilateraly, more so on the left. She is currently taking HRT and Vitamin E supplements, and reports one drink every evening. The patient admits to having her hair tinted because of some uneveness of color. Her urine sample fluoresces pink under a Wood's lamp. A preliminary diagnosis of porphyria cutanea tarda is reached, and order further studies are ordered. Which enzyme of heme synthesis is deficient in porphyria cutanea tarda?
  • δ-aminolevulinic acid synthase
  • Ferrochelatase
  • Porphobilinogen deaminase
  • Uroporphyrinogen III cosynthase
  • Uroporphyrinogen decarboxylase
 

·
Registered
Joined
·
487 Posts
I think its C) Porphobilinogen deaminase

A) & B) are associated with LEAD poisoning. Other two enzymes are not familiar to me. I don't really remember those enzyme. lol
 

·
Registered
Joined
·
487 Posts
oh my bad... I was thinking about the Acute intermittent porphyria. The enzyme that deficient in process is Porphobilinogen deaminase.

I think usmle0987 is correct.... :sorry:
 

·
Registered
Joined
·
590 Posts
Discussion Starter · #7 ·
Right!

Answer is E. Uroporphyrinogen decarboxylase (UROD) - deficient activity of this enzyme causes PCT, the most common subtype of porphyria.
  • δ-aminolevulinic acid (ALA) synthase deficiency (A) would inhibit heme synthesis entirely, causing a microcytic, hypochromic sideroblastic anemia. This can be caused by Vitamin B6 deficiency, among other things.
  • Porphobilinogen deaminase deficiency (C) causes acute intermittent porphyria. AIP does not usually manifest dermatologically; rather, patients present with abdominal pain, psychological symptoms, and peripheral neuropathy. AIP usually presents in middle adulthood.
  • Uroporphyrinogen III cosynthase deficiency (D) is the cause of congenital erythropoietic porphyria. This is a rare autosomal recessive disorder that usually presents in infancy or early adulthood.
  • Ferrochelatase deficiency (B) causes erythropoietic protoporphyria. This usually presents in childhood or early adulthood if congenital. It can also be caused by lead poisoning.
 

·
Registered
Joined
·
487 Posts
Let me put all those genetic diseases together in a simple diagram....


AIP: Acute intermittent porphyria; ALA: d-aminolevulinic acid; ALA-D: d-aminolevulinic acid dehydratase; CEP: congenital erythropoietic porphyria; COPROgen: coproporphyrinogen; CPGase: coproporphyrinogen oxidase; EPP: erythropoietic protoporphyria; Fe-Chel: ferrochelatase; HCP: hereditary coproporphyria; PBG: porphobilinogen; PBG-D: porphobilinogen deaminase; PCT: porphyria cutanea tarda; PPGase: protoporphyrinogen oxidase; PROTOgen: protoporphyrinogen; URO-D: uroporphyrinogen decarboxylase; UROgen: uroporphyrinogen; VP: variegate porphyria
 

·
Registered
Joined
·
201 Posts
Good Question and explanation

Thank you, I have been memorizing heme synthesis but couldn't. Now, I am pleased that I can digest it. Any idea, how to memorize syringomylin synthesis defect. My exam is next month, I still wrong the answer regarding these disease.
 

·
Registered
Joined
·
590 Posts
Discussion Starter · #10 ·
Thank you, I have been memorizing heme synthesis but couldn't. Now, I am pleased that I can digest it. Any idea, how to memorize syringomylin synthesis defect. My exam is next month, I still wrong the answer regarding these disease.
I think you mean the leukodystrophies, which are problems synthesizing myelin (sphingomyelin). Those are hard! I can usually remember Niemann-Pick and Krabbe's, but then I start getting confused with which are myelin disorders and which are lysosomal storage disorders...:( NEJM had an article last month that had this chart, which is pretty good but wordy:



Goljan (p.586 in 3e) only lists X-linked adrenoleukodystrophy, metachromatic, and Krabbe's. Given that you only have a short time, you might not want to dig too deeply into one thing...

Also, don't get sphingomyelin confused with syringomyelia, which is a cyst in the cervical spinal cord, often secondary to a Chiari type I malformation!



Good luck on your test, chulady! :happy:
 
1 - 10 of 10 Posts
Top