A 37-year-old woman comes to you for recurrent episodes of double vision and drooping of her eyelids for the last month. Such episodes occur, last for hours, and resolve spontaneously. She also reports occasional hoarseness, which also come and go. Vital signs and physical examination are normal. Most appropriate next step in diagnosis is?

**Core Concept**
Myasthenia gravis is an autoimmune disease characterized by weakness and rapid fatigue of the muscles under voluntary control. It is caused by antibodies against the acetylcholine receptor (AChR), leading to impaired neuromuscular transmission.

**Why the Correct Answer is Right**
The patient's symptoms of double vision, drooping eyelids (ptosis), and hoarseness are classic presentations of myasthenia gravis. The episodic nature of these symptoms, with periods of worsening followed by spontaneous resolution, is also consistent with this diagnosis. The antibodies against the AChR interfere with the normal release of acetylcholine at the neuromuscular junction, leading to muscle weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies such as CT or MRI of the brain may be useful in identifying structural abnormalities, but they are not the most appropriate next step in diagnosing myasthenia gravis.
**Option B:** Blood tests for Lyme disease may be useful in diagnosing other conditions that cause muscle weakness, but they are not directly relevant to this patient's symptoms.
**Option C:** Electromyography (EMG) may be useful in assessing muscle function, but it is not the most appropriate next step in diagnosing myasthenia gravis.

**Clinical Pearl / High-Yield Fact**
Myasthenia gravis is often associated with thymic abnormalities, and a significant proportion of patients have thymic hyperplasia or thymoma. This is an important consideration for patients with unexplained muscle weakness.

**Correct Answer:** C. Electromyography (EMG) to assess muscle function and possibly identify abnormalities consistent with myasthenia gravis.

✓ Correct Answer: D. Electromyography under repetitive stimulation