**Question:** All are aminoglycosides except:

A. Gentamicin
B. Streptomycin
C. Tobramycin
D. Neomycin

**Core Concept:** Aminoglycosides are a class of antibiotics commonly used for treating bacterial infections, particularly in Gram-negative bacteria. They work by binding to the 30S subunit of bacterial ribosomes, preventing protein synthesis and ultimately leading to bacterial cell death.

**Why the Correct Answer is Right:** D. Neomycin is not an aminoglycoside antibiotic. Neomycin belongs to the class of antibiotics known as bacteriostatic antibiotics, which inhibit bacterial growth rather than causing cell death. Aminoglycosides, on the other hand, are bactericidal antibiotics that directly interfere with bacterial protein synthesis.

**Why Each Wrong Option is Incorrect:**

A. Gentamicin: Correct answer, an aminoglycoside antibiotic used to treat various bacterial infections.
B. Streptomycin: Correct answer, another aminoglycoside antibiotic used to treat tuberculosis and other bacterial infections.
C. Tobramycin: Correct answer, an aminoglycoside antibiotic used to treat respiratory tract infections, urinary tract infections, and other bacterial infections.

**Why Neomycin is Incorrect:** Neomycin belongs to the bacteriostatic class of antibiotics, while the other options belong to the bactericidal class. Bacteriostatic antibiotics inhibit bacterial growth, preventing the bacteria from multiplying, but do not kill the bacteria. Bactericidal antibiotics, like aminoglycosides, directly interfere with bacterial cell components, causing cell death.

**Clinical Pearl:** Aminoglycoside antibiotics like gentamicin, streptomycin, and tobramycin are often used in combination therapy for treating severe bacterial infections. However, it is crucial to differentiate between the classes of antibiotics (bacteriostatic vs. bactericidal) to ensure appropriate treatment and avoid potential adverse effects from using inappropriate antibiotics.

**Correct Answer:** D. Neomycin