Locomotion And Movement

Disorders of the muscular and skeletal systems differ in their underlying causes, and recognising them requires linking symptoms to mechanisms. NCERT Class 11, Chapter 20 (Locomotion and Movement) lists myasthenia gravis as an autoimmune disorder affecting the neuromuscular junction, leading to fatigue, weakening and paralysis of skeletal muscles because antibodies impair signal transmission from nerve to muscle. Osteoporosis is incorrect; it is an age-related bone disorder marked by decreased bone mass and mineral density, commonly linked to reduced oestrogen levels, and it does not involve neuromuscular transmission. Gout is wrong because it results from the accumulation of uric acid crystals in joints, producing inflammation, and is a metabolic joint disorder rather than an autoimmune muscle disease. Tetany is also incorrect; it arises from a fall in blood calcium that causes rapid, wild muscle contractions, and is a calcium imbalance disorder, not an autoimmune attack on the junction. Therefore myasthenia gravis is the correct match. A mechanistic plausibility check confirms it: only an autoimmune attack on neuromuscular transmission explains both the immune basis and the progressive skeletal muscle weakness specified in the question.