**Core Concept**
Trimethoprim is a dihydrofolate reductase (DHFR) inhibitor, which plays a crucial role in the synthesis of tetrahydrofolate (THF). THF is essential for the synthesis of purines and pyrimidines, which are the building blocks of DNA.

**Why the Correct Answer is Right**
Trimethoprim acts by inhibiting the enzyme dihydrofolate reductase (DHFR), which is responsible for converting dihydrofolate (DHF) to tetrahydrofolate (THF). This inhibition prevents the synthesis of THF, which is necessary for the production of purines and pyrimidines. As a result, trimethoprim disrupts DNA synthesis and replication in rapidly dividing microorganisms, such as bacteria. This mechanism of action is particularly effective against organisms that are unable to synthesize THF from DHF, such as those that lack the enzyme dihydropteroate synthase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because trimethoprim does not act by inhibiting DNA gyrase, which is a target of other antibiotics such as ciprofloxacin.
* **Option B:** This option is incorrect because trimethoprim does not act by interfering with protein synthesis, which is a mechanism of action for antibiotics such as tetracycline.
* **Option C:** This option is incorrect because trimethoprim does not act by disrupting cell wall synthesis, which is a mechanism of action for antibiotics such as penicillin.

**Clinical Pearl / High-Yield Fact**
Trimethoprim is often used in combination with sulfamethoxazole (SMX) to enhance its antibacterial activity. This combination, known as co-trimoxazole, is effective against a broad spectrum of bacteria and is commonly used to treat urinary tract infections and other bacterial infections.

**Correct Answer: B. Inhibiting dihydrofolate reductase activity.**