## **Core Concept**
The question revolves around the treatment of infected foot lesions in a diabetic patient with a variety of opportunistic microbes. The focus is on the choice of antimicrobial agents, specifically which one must only be used topically.

## **Why the Correct Answer is Right**
Polymyxin B is an antibiotic that is effective against a wide range of Gram-negative bacteria. However, it is nephrotoxic and ototoxic when administered systemically. Due to these toxicities, **Polymyxin B is primarily used topically** (as eye drops or skin cream) or in certain cases via inhalation for infections that are resistant to other treatments. Its use is limited to topical application to avoid systemic toxicity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Mupirocin is an antibiotic that can be used topically but is not exclusively limited to topical use in all contexts; however, it's commonly used topically for skin infections.
- **Option B:** Bacitracin is another antibiotic that is commonly used topically, often in combination with other antibiotics like polymyxin B or neomycin. While primarily used topically due to nephrotoxicity with systemic use, the question asks for a drug that "must only be used topically," and while it implies exclusive topical use, there's nuance in its application.
- **Option D:** Neomycin is an aminoglycoside antibiotic that is used topically for skin infections. However, like bacitracin and polymyxin B, its systemic use is limited due to toxicity (ototoxicity and nephrotoxicity), but it's not exclusively used topically in a manner that's universally agreed upon as the correct answer here.

## **Clinical Pearl / High-Yield Fact**
A critical point to remember is that while several antibiotics can be used topically, **Polymyxin B** stands out due to its significant systemic toxicity profile, making topical use the safest and most appropriate route for its administration in many cases.

## **Correct Answer:** .