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: Ophthalmology
A patient using contact lens develops corneal infection. Laboratory diagnosis of acanthomoeba keratitis was established. The following is the best drug for treatment: 
Select and explain. 
A. Propamidine, B. Neosporine, C. Ketoconazole, D. Polyhexamethylene biguanide
D: Polyhexamethylene biguanide
Answer: D: Polyhexamethylene biguanide 
Explanation: D i.e. Polyhexamethylene biguanide