**Core Concept**
Myasthenia gravis is an autoimmune disease characterized by weakness and fatigue of skeletal muscles, resulting from impaired neuromuscular transmission. The underlying pathophysiology involves the production of autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction.

**Why the Correct Answer is Right**
The correct answer is D. Type II hypersensitivity. In myasthenia gravis, autoantibodies bind to the acetylcholine receptors on the postsynaptic muscle membrane, marking them for destruction by complement and leading to a decrease in the number of functional receptors. This results in impaired neuromuscular transmission and the characteristic symptoms of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Type I hypersensitivity is mediated by IgE antibodies and is responsible for allergic reactions such as anaphylaxis. It is not involved in myasthenia gravis.
**Option B:** Type III hypersensitivity involves the formation of antigen-antibody complexes that deposit in tissues and cause inflammation. This is not the primary mechanism of myasthenia gravis.
**Option C:** Type IV hypersensitivity is a cell-mediated reaction that involves T-lymphocytes and is responsible for delayed hypersensitivity reactions such as contact dermatitis. It is not involved in myasthenia gravis.

**Clinical Pearl / High-Yield Fact**
Myasthenia gravis often co-exists with thymoma, a type of tumor originating from the epithelial cells of the thymus. Approximately 10-15% of patients with myasthenia gravis have a thymoma, and the presence of a thymoma may influence the choice of treatment.

**Correct Answer:** D. Type II hypersensitivity reaction.