| Myasthenia gravis | Autoimmune | Neuromuscular junction | Anti-ACh receptor autoantibodies → insufficient depolarization of motor end plate | Fatigable skeletal muscle weakness; ptosis; dysphagia; worse with use | NMJ disorder; skeletal muscle only; distinct from muscular dystrophy |
| Muscular dystrophy | Genetic (X-linked) | Skeletal muscle fiber | Dystrophin gene mutation → fragile sarcolemma → progressive fiber death | Childhood onset; Gowers' sign; calf pseudohypertrophy; wheelchair by teens | Genetic NOT autoimmune; dystrophin NOT ACh receptor |
| Tetany | Metabolic | Peripheral nerves + muscles | Hypocalcemia → ↓ Na+ channel threshold → spontaneous nerve firing | Carpopedal spasm; Chvostek's sign; Trousseau's sign; paresthesias | Hypocalcemia is cause; distinct from bacterial tetanus |
| Osteoarthritis | Degenerative | Articular cartilage | Mechanical wear-and-tear → cartilage degradation → bone-on-bone contact | Elderly; weight-bearing joints; asymmetric; pain worse with activity | NOT autoimmune; no systemic features; cartilage (not synovium) |
| Rheumatoid arthritis | Autoimmune | Synovial membrane | T/B-cell attack → synovitis → pannus → cartilage and bone erosion | Symmetric small joints; morning stiffness >1hr; systemic features; anti-CCP | Autoimmune; synovial membrane; distinct from OA |
| Osteoporosis | Metabolic/Hormonal | Bone mass/density | Estrogen ↓ → osteoclast ↑ → resorption > formation → ↓ BMD | Post-menopausal women; fragility fractures (hip, wrist, vertebra) | Bone density disorder; estrogen connection; bisphosphonate treatment |
| Gout | Metabolic | Joint cavity (synovium) | Hyperuricemia → monosodium urate crystal deposition → acute inflammation | Big toe (podagra); sudden severe nocturnal pain; red, hot, swollen joint | Uric acid (purine catabolism end product); crystal-driven; diet-modifiable |