Dermatomyositis — SCE Dermatology MCQ

Correct answer: C — Anti-synthetase antibodies (e.g. anti-Jo-1)

The correct answer is C, Anti-synthetase antibodies (e.g. anti-Jo-1). Mechanic's hands, meaning fissuring and hyperkeratosis over the radial and palmar aspects of the fingers, is the classic cutaneous marker of antisynthetase syndrome, a distinct overlap phenotype of dermatomyositis driven by antibodies against aminoacyl-tRNA synthetases. Anti-Jo-1 (anti-histidyl-tRNA synthetase) is the most frequently detected antibody in this group, and the syndrome also features interstitial lung disease, non-erosive arthritis, Raynaud phenomenon and fever alongside myositis. The combination of myositis plus mechanic's hands should immediately trigger antisynthetase antibody testing and baseline pulmonary assessment, since interstitial lung disease is the main determinant of morbidity and mortality in this subgroup. Why the other options are wrong: B. Anti-SRP: this antibody defines an immune-mediated necrotising myopathy with severe, often treatment-resistant proximal weakness and high creatine kinase, but it is not linked to mechanic's hands or interstitial lung disease. E. Anti-TIF1-gamma: associated with classic dermatomyositis skin signs (Gottron papules, heliotrope rash) and is the antibody most strongly linked to underlying malignancy in adults, not mechanic's hands. A. Anti-Mi-2: associated with classic, relatively mild dermatomyositis with good treatment response and prominent cutaneous features such as the shawl sign, but not mechanic's hands. D. Anti-MDA5: characteristically causes rapidly progressive interstitial lung disease with often clinically amyopathic disease, plus skin ulceration and palmar papules; mechanic's hands can occur but are reported far less consistently than with antisynthetase antibodies. Key point: Mechanic's hands plus myositis should prompt testing for antisynthetase antibodies (anti-Jo-1 being the commonest) and screening for interstitial lung disease, which drives prognosis in antisynthetase syndrome.

Reference: DermNet NZ / British Association of Dermatologists: Dermatomyositis and myositis-specific autoantibodies, antisynthetase syndrome (anti-Jo-1); EULAR/ACR classification criteria for idiopathic inflammatory myopathies, 2017. https://dermnetnz.org/topics/dermatomyositis