**Core Concept**
Myasthenia gravis is an autoimmune neuromuscular disorder characterized by weakness and fatigue of skeletal muscles, resulting from antibodies against the acetylcholine receptor (AChR) at the neuromuscular junction. This leads to impaired transmission of nerve impulses to muscles.

**Why the Correct Answer is Right**
The correct answer is D, as myasthenia gravis typically presents with fluctuating muscle weakness that worsens with repeated use (fatigability) and improves with rest. This is due to the progressive destruction of AChR by autoantibodies, reducing the number of functional receptors available for neurotransmitter binding. As a result, patients often exhibit ptosis (drooping eyelids), diplopia (double vision), and difficulty swallowing or speaking.

**Why Each Wrong Option is Incorrect**
**Option A:** Muscle cramps are not typically associated with myasthenia gravis, which usually presents with weakness rather than cramping or spasms.

**Option B:** Weakness of proximal limb muscles is a hallmark of myasthenia gravis, as the disease often affects the muscles responsible for movement and support of the arms and legs.

**Option C:** Fatigue and weakness of the diaphragm and other respiratory muscles can occur in myasthenia gravis, potentially leading to respiratory failure in severe cases.

**Clinical Pearl / High-Yield Fact**
Myasthenic crisis, a life-threatening complication of myasthenia gravis, requires immediate medical attention due to its potential for respiratory failure and requires aggressive treatment with immunosuppressive medications and mechanical ventilation.

**Correct Answer: D. Muscle cramps.**