**Core Concept**
Inactivating enzymes, also known as antibiotic-modifying enzymes, are a key mechanism of antibiotic resistance in bacteria. These enzymes can inactivate or modify antibiotics, rendering them ineffective against the bacteria. This mechanism is particularly common in Gram-negative bacteria.

**Why the Correct Answer is Right**
The correct answer is A. This is because beta-lactamase enzymes, which inactivate beta-lactam antibiotics like penicillins and cephalosporins, are a classic example of inactivating enzymes. These enzymes work by hydrolyzing the beta-lactam ring of the antibiotic, rendering it ineffective. Other examples of inactivating enzymes include aminoglycoside acetyltransferases, which inactivate aminoglycoside antibiotics.

**Why Each Wrong Option is Incorrect**
* **Option B:** Aminoglycoside resistance can also be due to efflux pumps, which actively remove the antibiotic from the bacterial cell, but inactivating enzymes like aminoglycoside acetyltransferases are also a key mechanism.
* **Option C:** Methylation of bacterial ribosomal RNA can confer resistance to aminoglycoside antibiotics, but this is not an example of inactivating enzymes.
* **Option D:** Mutations in the target of the antibiotic, such as the ribosome or DNA gyrase, can also confer resistance, but this is not an example of inactivating enzymes.

**Clinical Pearl / High-Yield Fact**
Inactivating enzymes are often plasmid-encoded, meaning they can be easily transferred between bacteria, making them a major concern for public health.

**Correct Answer: D. Methylation of bacterial ribosomal RNA can confer resistance to aminoglycoside antibiotics, but this is not an example of inactivating enzymes.**