This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A Thirty five year old female has proximal weakness of muscles, ptosis and easy fatigability. The most sensitive test to suggest the diagnosis is - 
Select and explain. 
A. Muscle Biopsy, B. CPK levels, C. Edrophonium test, D. Single fiber EMG
D: Single fiber EMG
Answer: D: Single fiber EMG 
Explanation: None