WBR253

Author	[[PageAuthor::Gerald Chi (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	USMLE Step 1
Main Category	Biochemistry, Pathophysiology
Sub Category	Musculoskeletal/Rheumatology
Prompt	A 37-year-old woman presents with malaise, proxmial muscle weakness, and swollen hand and knee joints. She also reports dry eyes and dry mouth. When exposed to cold temperatures, her fingers become pale and cyanotic and turn red and numb. Physical examination is remarkable for scaly erythematous eruptions overlying the knuckles and elbows. Laboratory findings demonstrate lymphopenia with mild anemia and thrombocytopenia. A diagnosis is then made following the detection of speckled anti-nuclear antibody as well as autoantibodies against extractable nuclear antigens. Which of the following structures is most likely to be the target of the antibodies?
Answer A	Transfer RNA (tRNA)
Answer A Explanation	Anti-U1-RNP antibody does not target tRNA.
Answer B	Small interfering RNA (siRNA)
Answer B Explanation	Anti-U1-RNP antibody does not target siRNA
Answer C	Small nuclear ribonucleoprotein (snRNP)
Answer C Explanation	Anti-U1-RNP antibody targets snRNP, which is a complex of snRNA (produced by RNAP II) and multiple proteins.
Answer D	Heterogeneous nuclear RNA (hnRNA)
Answer D Explanation	Anti-U1-RNP antibody does not target hnRNA. The snRNP combines with hnRNA to form spliceosome and undergoes splicing.
Answer E	Deoxyribonucleic acid (DNA)
Answer E Explanation	Anti-U1-RNP antibody does not target DNA.
Right Answer	C
Explanation	Mixed connective tissue disease (MCTD) combines features of scleroderma, myositis, systemic lupus erythematosus, and rheumatoid arthritis and is thus considered an overlap syndrome. MCTD commonly manifests with joint pain/swelling, malaise, Raynaud phenomenon, Sjögren's syndrome, muscle inflammation, and sclerodactyly. Distinguishing laboratory characteristics are a positive, speckled anti-nuclear antibody and an anti-U1-RNP antibody. Educational Objective: Anti-U1-RNP antibodies are often elevated in patients with mixed connective tissue disease. Anti-U1-RNP antibody targets snRNP, which is a complex of snRNA (produced by RNAP II) and multiple proteins. References: Ghirardello A, Doria A, Vesco P, et al. Blotting patterns of IgG anti-(U1)RNP antibodies in mixed connective tissue disease. Rheumatol Int. 1996;16(4):145-50. First Aid 2015 page 213.
Approved	Yes
Keyword	Anti-U1-RNP antibody, Mixed connective tissue disease, MCTD, Small nuclear ribonucleoprotein
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