Which condition is characterized by muscle weakness due to the immune system attacking muscle cells?

Myasthenia gravis is the condition characterized by muscle weakness resulting from the immune system's attack on acetylcholine receptors at the neuromuscular junction. This autoimmune disorder disrupts communication between nerves and muscles, leading to weakness that often worsens with activity and improves with rest. The hallmark of myasthenia gravis is its fluctuating nature; patients may experience varying degrees of weakness in different muscle groups, commonly affecting the eye muscles, facial muscles, and limbs.

In contrast, the other conditions listed do not involve the immune system directly attacking muscle cells. Osteoporosis primarily affects bone density and strength, rheumatoid arthritis involves chronic inflammation of the joints, and torticollis pertains to abnormal neck positioning due to muscle contractions. Each of these conditions has different underlying mechanisms and symptoms that do not involve the immune system's direct assault on muscle cells, as seen in myasthenia gravis.