**Core Concept**
Myasthenia gravis (MG) is a chronic autoimmune disorder characterized by weakness and rapid fatigue of the muscles under voluntary control. The disease is caused by antibodies targeting the acetylcholine receptor (AChR) at the neuromuscular junction, impairing neurotransmitter release and muscle contraction.

**Why the Correct Answer is Right**
The single-fiber electromyography (EMG) is considered the most sensitive test for diagnosing myasthenia gravis. This test evaluates the transmission of electrical signals between nerve and muscle fibers, allowing for the detection of even slight abnormalities in neuromuscular transmission. The presence of jitter, blocking, or other abnormalities in single-fiber EMG can confirm the diagnosis of MG.

**Why Each Wrong Option is Incorrect**
**Option A:** Edrophonium test, also known as Tensilon test, is a diagnostic test for myasthenia gravis but is less sensitive than single-fiber EMG. It involves administering edrophonium chloride, a cholinesterase inhibitor, to temporarily improve muscle strength in MG patients.

**Option B:** Repetitive nerve stimulation (RNS) is a diagnostic test for myasthenia gravis but is less sensitive than single-fiber EMG. RNS involves repetitive stimulation of a nerve to assess the response of the muscle.

**Option C:** Blood tests for anti-acetylcholine receptor (anti-AChR) antibodies are used to support the diagnosis of myasthenia gravis but are not as sensitive as single-fiber EMG.

**Clinical Pearl / High-Yield Fact**
Single-fiber EMG can detect subtle abnormalities in neuromuscular transmission, making it the most sensitive test for diagnosing myasthenia gravis. Remember that a positive single-fiber EMG can confirm the diagnosis, even in the absence of other diagnostic tests.

**Correct Answer:** C. Blood tests for anti-acetylcholine receptor (anti-AChR) antibodies.