The most sensitive test for diagnosis of myasthenia gravis is
**Core Concept**
Myasthenia gravis (MG) is an autoimmune disease characterized by the production of antibodies against the acetylcholine receptor (AChR), leading to muscle weakness and fatigue. The diagnosis of MG requires a sensitive test that can detect these antibodies.
**Why the Correct Answer is Right**
The most sensitive test for diagnosing myasthenia gravis is the **single-fiber electromyography (SFEMG)**. SFEMG measures the transmission of nerve impulses to muscle fibers and can detect even slight abnormalities in neuromuscular transmission. This is particularly useful in early MG, where muscle strength may still be within normal limits. The SFEMG test is highly sensitive for detecting AChR antibodies, which are the hallmark of MG.
**Why Each Wrong Option is Incorrect**
* **Option A:** The **Edrophonium test**, also known as the Tensilon test, is an older test for MG that involves administering edrophonium chloride, a cholinesterase inhibitor. While it can produce temporary improvements in muscle strength, it is less sensitive than SFEMG and can have false-positive results.
* **Option B:** **Serum acetylcholine receptor antibody (S-AChR Ab) testing** is a useful diagnostic tool for MG, but it is not as sensitive as SFEMG, particularly in early disease.
* **Option D:** **Muscle biopsy** may show changes consistent with MG, but it is not a sensitive test for diagnosing the condition.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that SFEMG is the most sensitive test for diagnosing myasthenia gravis, especially in early disease. This is because it can detect even slight abnormalities in neuromuscular transmission, which is critical for making an accurate diagnosis.
**Correct Answer:** C. Single-fiber electromyography (SFEMG) is the most sensitive test for diagnosing myasthenia gravis.