**Core Concept:** Nonsteroidal Anti-Inflammatory Drugs (NSAIDs) are a class of medications used for pain relief and inflammation management. They work by inhibiting the enzyme cyclooxygenase (COX) and blocking the production of prostaglandins, which are involved in inflammation, pain, and fever.

**Why the Correct Answer is Right:** The longest acting NSAID among the options is **C.** Diclofenac (Voltaren). Diclofenac is a selective COX-2 inhibitor, meaning it preferentially targets the COX-2 enzyme, leading to a longer duration of action compared to other NSAIDs. COX-2 plays a role in inflammation, pain, and fever, and its inhibition results in reduced prostaglandin production.

**Why Each Wrong Option is Incorrect:**

A. **Meloxicam (Mobic):** Though it is a COX-2 inhibitor, it has some COX-1 inhibition as well, resulting in a shorter duration of action compared to diclofenac.

B. **Ibuprofen (Advil, Nurofen):** Ibuprofen is a non-selective NSAID, meaning it inhibits both COX-1 and COX-2 enzymes. Its shorter duration of action is due to its non-selective COX inhibition.

D. **Aceclofenac (Clofen):** Aceclofenac is a selective COX-2 inhibitor, but its duration of action is shorter than diclofenac due to its non-selective COX inhibition.

**Clinical Pearl:** Understanding the selectivity of NSAIDs for COX-1 and COX-2 enzymes is essential for proper drug selection based on patient's condition and desired duration of action.

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**Correct Answer:** C. Diclofenac (Voltaren)

Diclofenac is the longest acting NSAID due to its selectivity for COX-2 inhibition. This results in reduced off-target effects and a longer duration of action compared to other options.