**Core Concept:** Antibiotics are substances that inhibit the growth or activity of microorganisms, particularly bacteria. Sabouraud's dextrose agar is a culture medium used for the isolation and identification of fungi, particularly dermatophytes, and is often contaminated with bacteria. Adding antibiotics to the medium helps prevent the growth of unwanted bacteria while allowing fungal colonies to grow.

**Why the Correct Answer is Right:** The correct answer, chloramphenicol, is a broad-spectrum antibacterial agent that inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, preventing peptide bond formation. It is added to Sabouraud's dextrose agar to inhibit the growth of bacteria contaminating the agar, allowing fungal colonies to grow uninhibited for proper identification and differentiation.

**Why Each Wrong Option is Incorrect:**

A. Neomycin (Option A) is a broad-spectrum antibiotic that inhibits bacterial protein synthesis, similar to chloramphenicol. However, it does not have as strong a bacteriostatic effect as chloramphenicol and is therefore less effective in inhibiting bacterial growth on the agar.

B. Streptomycin (Option B) is another antibiotic that inhibits bacterial protein synthesis. It is less effective than chloramphenicol, making it an unsuitable choice for inhibiting bacteria on the agar.

C. Gramicidin (Option C) is an antibiotic that also inhibits bacterial protein synthesis. However, it has a narrower spectrum of activity and is less effective than chloramphenicol for inhibiting bacteria on the agar.

D. Amphotericin B (Option D) is an antifungal agent used to inhibit fungal growth. It is not suitable for inhibiting bacterial growth on the agar, making it an incorrect choice.

**Clinical Pearl:** Chloramphenicol is a preferred choice for inhibiting bacterial growth on Sabouraud's dextrose agar due to its strong bacteriostatic effect on bacterial protein synthesis, allowing fungal colonies to grow and be easily identifiable.