**Core Concept**
Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disorder characterized by muscle weakness and fatigue. It is caused by antibodies against the voltage-gated calcium channels (VGCCs) at the neuromuscular junction, leading to impaired neurotransmitter release and muscle weakness.

**Why the Correct Answer is Right**
In LEMS, autoantibodies target the P/Q-type VGCCs, specifically the alpha-1 subunit, which is essential for neurotransmitter release from the presynaptic neuron. This results in decreased acetylcholine release, leading to muscle weakness and fatigue. The antibodies also bind to the VGCCs in the autonomic nervous system, causing symptoms such as dry mouth, constipation, and impotence.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because LEMS is not caused by an abnormality in the acetylcholine receptors, but rather by an autoimmune response against the voltage-gated calcium channels.
**Option B:** This option is incorrect because LEMS is not characterized by muscle cramps, which are more typical of neuromuscular junction disorders such as myasthenia gravis.
**Option C:** This option is incorrect because LEMS is not typically associated with muscle atrophy, which is more characteristic of motor neuron diseases such as amyotrophic lateral sclerosis.

**Clinical Pearl / High-Yield Fact**
A key feature of LEMS is the improvement in muscle strength after repeated nerve stimulation, a phenomenon known as "post-exercise facilitation." This is due to the increased release of neurotransmitters from the remaining functioning VGCCs.

**Correct Answer: C. LEMS is characterized by muscle weakness and fatigue that improves with repeated nerve stimulation.**