**Core Concept**
Cyclooxygenase (COX) is an enzyme involved in the synthesis of prostaglandins from arachidonic acid. Prostaglandins play a crucial role in mediating inflammation, pain, and fever. There are two isoforms of COX: COX-1, which is constitutively expressed, and COX-2, which is inducibly expressed in response to inflammatory stimuli.

**Why the Correct Answer is Right**
Aspirin acts as an irreversible inhibitor of COX by acetylating a serine residue in the active site of the enzyme, resulting in the formation of a stable acetyl-CoA intermediate. This blocks the enzyme's activity, preventing the conversion of arachidonic acid to prostaglandin H2 and subsequently to other prostaglandins. The irreversible inhibition of COX by aspirin leads to a decrease in prostaglandin synthesis, which in turn reduces pain, inflammation, and fever.

**Why Each Wrong Option is Incorrect**
**Option A:** Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen are reversible inhibitors of COX, not irreversible.
**Option B:** Acetaminophen is a weak inhibitor of COX-3, but it is not an irreversible inhibitor.
**Option C:** Indomethacin is a reversible inhibitor of COX, not irreversible.

**Clinical Pearl / High-Yield Fact**
Aspirin's mechanism of action is unique among NSAIDs due to its irreversible inhibition of COX. This characteristic leads to a prolonged effect, even after the drug is discontinued.

**Correct Answer:** A. Aspirin.