Which condition is described as a systemic rheumatic disorder involving skin and muscle changes?

Dermatomyositis is characterized as a systemic rheumatic disorder that primarily affects the skin and muscles. This condition manifests with distinctive skin rashes and muscle weakness, making it unique compared to other rheumatic disorders. The skin changes can include heliotrope rash (a purplish rash), Gottron's papules (rash on the knuckles), and other lesions, while muscle involvement typically presents as weakness, especially in the proximal muscles.

In contrast, other conditions listed do not share the same systemic features or specific skin changes associated with dermatomyositis. Osteoarthritis mainly impacts the joints and does not typically involve skin or muscle symptoms. Myasthenia gravis is primarily a neuromuscular disorder affecting muscle strength without significant skin involvement. Rheumatoid arthritis is more focused on joint inflammation and may not include the distinct muscle and skin changes seen in dermatomyositis. Thus, dermatomyositis is the correct answer due to its specific combination of systemic effects on skin and muscle.