Which condition involves antibodies against acetylcholine receptors?

Myasthenia gravis is characterized by the presence of antibodies that target acetylcholine receptors at the neuromuscular junction. This autoimmune condition leads to a reduction in the number of functional acetylcholine receptors, impairing the ability of acetylcholine to stimulate muscle contraction. As a result, individuals with myasthenia gravis experience muscle weakness and fatigue that worsens with activity and improves with rest.

In contrast, conditions such as multiple sclerosis involve immune-mediated damage to the central nervous system, primarily affecting myelin rather than acetylcholine receptors. Lupus erythematosus is a systemic autoimmune disease that can affect multiple organ systems but does not specifically involve antibodies against acetylcholine receptors. Guillain-Barré syndrome is an acute inflammatory demyelinating polyneuropathy primarily affecting peripheral nerves, and while it does involve an autoimmune mechanism, it does not target acetylcholine receptors.

These distinctions make myasthenia gravis the correct answer, given its direct relationship with antibodies against acetylcholine receptors.