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Metalloproteinase-mediated protein degradation
38%
104/274
Nucleotide excision repair
1%
2/274
Platelet binding to fibrinogen
19%
51/274
Platelet binding to von Willebrand factor
31%
85/274
Porphobilinogen metabolism
7%
20/274
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The patient in this vignette presents with anemia, thrombocytopenia, splenomegaly and confusion suggestive of thrombotic thrombocytopenic purpura (TTP). TTP is caused by a defect in ADAMTS13, a metalloproteinase that degrades von Willebrand factor (vWF) multimers into vWF monomers. TTP is a thrombotic microangiopathy characterized by a pentad of microangiopathic hemolytic anemia, acute renal failure, thrombocytopenia, fever, and neurologic abnormalities. It is caused by a deficiency or inhibition of ADAMTS13, a metalloproteinase that normally functions to degrade vWF multimers into functional monomers. The buildup of vWF multimers leads to increased platelet adhesion and thrombosis. Incorrect Answers: Answer 2: Nucleotide excision repair is a type of DNA repair. Xeroderma pigmentosum is a genetic disorder characterized by hypersensitivity to ultraviolet light that is caused by defective nucleotide excision repair. Answer 3: Platelet binding to fibrinogen is mediated by the GpIIb/IIIa receptor. Glanzmann thrombasthenia is an inherited platelet disorder caused by a deficiency in the GpIIb/IIIa receptor. This receptor is on the surface of platelets and binds to fibrinogen, allowing multiple platelets to bind to the same fibrinogen molecule and promoting platelet aggregation. A deficiency in the GpIIb/IIIa receptor therefore leads to impaired platelet aggregation. Clinically, it presents with prolonged bleeding time and easy bleeding. Answer 4: Platelet binding to von Willebrand factor is mediated by the GP1b receptor. Bernard-Soulier disease is an inherited platelet disorder caused by a deficiency in the GP1b receptor. This receptor is on the surface of platelets and binds to vWF on the endothelial surface. A deficiency in GP1b subsequently leads to impaired platelet adhesion. Clinically, it presents with thrombocytopenia and easy bleeding. Answer 5: Porphobilinogen metabolism is an important part of the heme synthesis pathway. Acute intermittent porphyria is a metabolic disease caused by defective porphobilinogen deaminase, an enzyme in the heme synthesis pathway that converts porphobilinogen to hydroxymethylbilane. Clinically, it presents with abdominal pain, neurological symptoms, psychiatric symptoms, and port wine-colored urine. Bullet Summary: TTP is caused by a defect in ADAMTS13, a metalloproteinase that cleaves vWF multimers into vWF monomers.
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