A 25-year-old female presents with fever, joint pain, and a butterfly rash on the cheeks
Question : A 25-year-old female presents with fever, joint pain, and a butterfly rash on the cheeks. Which of the following autoantibodies is most specific for this disease?
A. Antinuclear antibody (ANA)
B. Anti-double-stranded DNA (dsDNA) antibody
C. Anti-Smith (Sm) antibody
D. Anti-ribonucleoprotein (RNP) antibody
Answer: C
Explanation: The patient's manifestations (fever, joint pain, butterfly rash) are typical of systemic lupus erythematosus (SLE). Anti-Smith (Sm) antibody is highly specific for SLE (specificity >99%), although its sensitivity is low. ANA is a screening antibody but not specific; anti-dsDNA antibody is specific but less than anti-Sm antibody; anti-RNP antibody is associated with mixed connective tissue disease. Therefore, the correct answer is C.
