**Core Concept**
The question requires the identification of a beta-lactam antibiotic, which is a class of antibiotics that contain a beta-lactam ring in their molecular structure. Beta-lactam antibiotics work by inhibiting bacterial cell wall synthesis, leading to cell lysis and death.

**Why the Correct Answer is Right**
Beta-lactam antibiotics include penicillins (e.g., ampicillin), cephalosporins (e.g., ceftriaxone), monobactams (e.g., aztreonam), and carbapenems (e.g., imipenem). These antibiotics all have a beta-lactam ring in their structure, which is essential for their mechanism of action. The correct answer will be a class of antibiotics that does not have a beta-lactam ring.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillins are a class of beta-lactam antibiotics that work by inhibiting bacterial cell wall synthesis. They contain a beta-lactam ring in their structure.

**Option B:** Cephalosporins are a class of beta-lactam antibiotics that work by inhibiting bacterial cell wall synthesis. They contain a beta-lactam ring in their structure.

**Option C:** Monobactams are a class of beta-lactam antibiotics that work by inhibiting bacterial cell wall synthesis. They contain a beta-lactam ring in their structure.

**Option D:** Aminoglycosides (e.g., gentamicin) are a class of antibiotics that work by inhibiting bacterial protein synthesis. They do not have a beta-lactam ring in their structure, making them the correct answer.

**Clinical Pearl / High-Yield Fact**
Beta-lactam antibiotics are commonly used to treat a wide range of bacterial infections, including respiratory tract infections, skin infections, and urinary tract infections. They are often used in combination with other antibiotics to enhance their effectiveness.

**Correct Answer:** D. Aminoglycosides.