**Core Concept**
The diagnosis of myasthenia gravis, an autoimmune disorder characterized by muscle weakness and fatigue, often requires a sensitive diagnostic test to confirm the presence of neuromuscular transmission defects. Single fiber electromyography (EMG) is a specialized test that evaluates the communication between nerves and muscles.

**Why the Correct Answer is Right**
Single fiber EMG is highly sensitive for detecting neuromuscular junction dysfunction, making it an ideal test for diagnosing myasthenia gravis. This test measures the time it takes for a single muscle fiber to contract and relax in response to a nerve impulse, allowing for the assessment of the strength and reliability of neuromuscular transmission. In myasthenia gravis, the nerve-muscle communication is impaired, resulting in delayed or abnormal muscle fiber contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** Muscle Biopsy may show inflammation or atrophy of muscle fibers, but it is not as sensitive as single fiber EMG for detecting neuromuscular transmission defects.
**Option B:** Elevated CPK (creatine phosphokinase) levels are indicative of muscle damage, but they are not specific to myasthenia gravis and can be seen in various muscle disorders.
**Option C:** The Edrophonium test, also known as the Tensilon test, involves the administration of a cholinesterase inhibitor to temporarily improve muscle strength in patients with myasthenia gravis. However, this test is less sensitive and more subjective than single fiber EMG, and it carries a risk of adverse reactions.

**Clinical Pearl / High-Yield Fact**
Single fiber EMG is particularly useful for diagnosing myasthenia gravis in patients with mild symptoms or in those who have a negative acetylcholine receptor antibody test. A normal single fiber EMG result can help rule out myasthenia gravis, while an abnormal result supports the diagnosis.

**✓ Correct Answer: D. Single fiber EMG**