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: Pharmacology
Strength of topical ophthalmic preparations of tobramycin is : September 2009 
Select and explain. 
A. 3 mg/ ml, B. 8 mg/ ml, C. 10 mg/ml, D. 13 mg/ml
A: 3 mg/ ml
Answer: A: 3 mg/ ml 
Explanation: Ans. A: 3 mg/ml Tobramycin works by binding to a site on the bacterial 30S and 50S ribosome, preventing formation of the 70S complex. As a result, mRNA cannot be translated into protein and cell death ensues. Sterile Tobramycin Ophtha Solution (eye-drops) contains tobramycin concentration of 0.3%. It is mixed with 0.01% benzalkonium chloride as a preservative. This concentrations result in 3 mg per ml.