In which condition does the immune system mistakenly attack neuromuscular junctions, leading to weakness?

Myasthenia gravis is characterized by the immune system's unwanted attack on the neuromuscular junctions, which are critical for communication between nerves and muscles. In this condition, antibodies interfere with the receptors for acetylcholine, a neurotransmitter necessary for muscle contraction. As a result, patients experience muscle weakness and fatigue, particularly after periods of activity.

This specific mechanism distinguishing myasthenia gravis from other conditions makes it the correct answer. In contrast, multiple sclerosis primarily affects the central nervous system and involves the demyelination of nerve fibers, leading to neurological symptoms rather than direct attacks on the neuromuscular junction. Lupus and rheumatoid arthritis are systemic autoimmune conditions that lead to systemic inflammation and impact various tissues and organs, but do not specifically target the neuromuscular junctions in the same way that myasthenia gravis does. Thus, the unique action of myasthenia gravis on the neuromuscular junction is what firmly identifies it as the correct answer in this context.