**Core Concept**
Myasthenia gravis (MG) is an autoimmune disease characterized by weakness and rapid fatigue of the muscles under voluntary control. The fundamental defect in MG lies at the neuromuscular junction, where the transmission of nerve impulses to muscles is impaired.

**Why the Correct Answer is Right**
In MG, autoantibodies target the nicotinic acetylcholine receptors (nAChRs) on the postsynaptic muscle membrane. These autoantibodies cause a reduction in the number of functional nAChRs, leading to decreased acetylcholine binding and subsequent muscle contraction. As a result, the nerve impulses are not transmitted effectively to the muscles, causing weakness and fatigue. The impaired neuromuscular transmission is due to the decreased density of nAChRs and the presence of autoantibodies that block or modulate the receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not specify the neuromuscular junction, which is the site of the fundamental defect in MG.
**Option B:** While acetylcholine is the neurotransmitter involved in neuromuscular transmission, the fundamental defect in MG is not the lack of acetylcholine itself but rather the impaired binding of acetylcholine to its receptors.
**Option C:** This option is vague and does not specify the neuromuscular junction or the nicotinic acetylcholine receptors.

**Clinical Pearl / High-Yield Fact**
The presence of thymic hyperplasia or thymoma is a common association with MG, and surgical removal of the thymus may improve symptoms in some patients.

**Correct Answer: C. Decreased density of nicotinic acetylcholine receptors.**