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MCTD Disease Evolution — SCE Rheumatology MCQ

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ModerateConnective Tissue DiseaseMCTD Disease EvolutionSCE Rheumatology

A 45-year-old woman initially fulfilled accepted criteria for mixed connective tissue disease (MCTD), with Raynaud phenomenon, puffy fingers, inflammatory arthritis, myositis and strongly positive anti-U1 RNP antibodies. She did not initially fulfil classification criteria for another defined connective tissue disease. Over the next 5 years, she develops progressive sclerodactyly, fingertip pitting scars, telangiectasia and interstitial lung disease and now fulfils classification criteria for systemic sclerosis. What is the best interpretation of this clinical course?

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Correct answer: A — Phenotypic conversion from MCTD to a differentiated connective tissue disease, prompting reassessment

The correct answer is A. MCTD may remain phenotypically stable, but a clinically important minority of patients subsequently fulfil criteria for a differentiated connective tissue disease, most often systemic sclerosis or SLE. Contemporary cohorts report conversion in approximately one-quarter to one-third of patients, depending on cohort definitions and follow-up. Her new sclerodactyly, pitting scars, telangiectasia and ILD establish a dominant systemic-sclerosis phenotype and warrant diagnostic, organ-screening and treatment reassessment. This does not prove that the original MCTD diagnosis was wrong, because genuine longitudinal conversion can occur. Anti-U1 RNP supports MCTD but does not itself cause or define conversion to systemic sclerosis. Progression is not inevitable; most patients in longitudinal cohorts do not convert.

Reference: RMD Open. Unravelling IPAF, VEDOSS and connective tissue diseases classifications through the mixed connective tissue disease spectrum. 2025. https://pubmed.ncbi.nlm.nih.gov/41130746/