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 repos occasional hoarseness, which also come and go. Vital signs and physical examination are normal. Most appropriate next step in diagnosis is?

**Core Concept**
The patient's symptoms suggest a disorder affecting the cranial nerves, specifically the third (oculomotor) and seventh (facial) nerves, which are responsible for controlling eye movements and eyelid function. The involvement of multiple cranial nerves points towards a condition that affects the brainstem or a peripheral nerve.

**Why the Correct Answer is Right**
The patient's symptoms are suggestive of **Myasthenia Gravis (MG)**, an autoimmune disease characterized by weakness and rapid fatigue of the muscles under voluntary control. In MG, antibodies target the **nicotinic acetylcholine receptors (nAChR)** on the postsynaptic membrane of the neuromuscular junction, disrupting acetylcholine-mediated neurotransmission. This leads to fluctuations in muscle strength, particularly affecting the cranial nerves responsible for eye movements and eyelid function. The episodic nature of the patient's symptoms, with hours-long episodes of double vision and drooping eyelids, is consistent with MG.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies like MRI or CT scans may not be the most appropriate next step, as they may not reveal the underlying cause of the patient's symptoms.

**Option B:** Blood work may be helpful in diagnosing MG, but it is not the most appropriate next step at this stage, as the patient's symptoms suggest a more urgent need for a definitive diagnosis.

**Option C:** Electromyography (EMG) is a useful diagnostic tool for MG, but it is usually performed after a clinical diagnosis has been established or a trial of anticholinesterase medications has been initiated.

**Option D:** A trial of anticholinesterase medications like pyridostigmine is a reasonable next step, as it can help confirm the diagnosis of MG and improve the patient's symptoms.

**Clinical Pearl / High-Yield Fact**
MG is a treatable condition, and prompt diagnosis can significantly improve patient outcomes. A key clinical feature of MG is the improvement of symptoms with rest and the exacerbation of symptoms with prolonged activity.

**Correct Answer:** D.

✓ Correct Answer: D. Electromyography under repetitive stimulation