According to the text, Arthritis is correctly defined as the 'Inflammation of joints'. The other options are incorrect matches based on the provided definitions:

• Myasthenia gravis is an auto-immune disorder (not genetic) affecting the neuromuscular junction.
• Muscular dystrophy is a progressive degeneration of skeletal muscle mostly due to a genetic disorder (not auto-immune).
• Tetany is caused by low Ca2+ levels in body fluid (not high).