**Question:** The "angry back" phenomenon is related to ONE of the following:

A. Acetaminophen
B. Aspirin
C. Nonsteroidal anti-inflammatory drugs (NSAIDs)
D. Antacids

**Correct Answer:** C. Nonsteroidal anti-inflammatory drugs (NSAIDs)

**Core Concept:** The "angry back" phenomenon is a term used to describe the phenomenon of increased back pain in patients taking nonsteroidal anti-inflammatory drugs (NSAIDs). This side effect is a result of the drug's action on the cyclooxygenase (COX) enzymes, which are responsible for the synthesis of prostaglandins.

**Why the Correct Answer is Right:** NSAIDs inhibit the cyclooxygenase enzymes (COX-1 and COX-2) responsible for the synthesis of prostaglandins. Prostaglandins are involved in maintaining the integrity of the gastrointestinal mucosa through the protective actions of prostaglandin E2 (PGE2). When COX enzymes are inhibited by NSAIDs, this results in decreased production of PGE2, leading to increased sensitivity of the gastrointestinal mucosa and an increased risk of mucosal damage, ulceration, and bleeding. This effect is most prominent in the elderly population and those with pre-existing gastrointestinal disorders.

**Why Each Wrong Option is Incorrect:**

A. Acetaminophen (paracetamol): Acetaminophen is a centrally acting analgesic and antipyretic drug. It does not inhibit COX enzymes and thus does not cause the "angry back" phenomenon.

B. Aspirin: Aspirin is a nonselective NSAID that inhibits both COX-1 and COX-2 enzymes, leading to the same side effects as NSAIDs. However, aspirin is not commonly associated with the "angry back" phenomenon.

D. Antacids: Antacids are used to neutralize stomach acid and relieve symptoms of gastrointestinal disorders, but they do not cause the "angry back" phenomenon.

**Clinical Pearl:** The "angry back" phenomenon is a clinical warning sign that highlights the potential gastrointestinal complications associated with NSAID use. It is crucial for healthcare providers to consider this side effect when prescribing NSAIDs, especially in high-risk patients, to prevent unnecessary suffering and complications.