**Core Concept**
Topical ophthalmic preparations of tobramycin are a type of aminoglycoside antibiotic used to treat bacterial infections of the eye. Tobramycin is a water-soluble antibiotic that requires a specific concentration to effectively treat ocular infections without causing toxicity.

**Why the Correct Answer is Right**
The correct concentration of tobramycin in topical ophthalmic preparations is 3 mg/mL. This is because higher concentrations can cause toxicity to the corneal epithelium, leading to side effects such as corneal ulcers and vision loss. The 3 mg/mL concentration is effective in treating bacterial infections of the eye while minimizing the risk of toxicity. Tobramycin works by inhibiting protein synthesis in bacteria, which is mediated by the enzyme ribosome.

**Why Each Wrong Option is Incorrect**
**Option B:** 8 mg/mL is too high and can cause corneal toxicity, making it an incorrect option.
**Option C:** 10 mg/mL is also excessive and can lead to ocular side effects, making it an incorrect choice.
**Option D:** 13 mg/mL is significantly higher than the recommended concentration and would likely cause severe toxicity, making it an incorrect option.

**Clinical Pearl / High-Yield Fact**
Remember that aminoglycoside antibiotics, including tobramycin, can cause ototoxicity and nephrotoxicity when used systemically, but topical ophthalmic preparations are generally safe when used at the recommended concentration.

**Correct Answer Line**
✓ Correct Answer: A. 3 mg/ ml