A 54-year-old man comes to the emergency room with severe pain in his right toe. He has had less severe episodes in the past, which he always treated with pain medications. The toe is red, inflamed, and exquisitely sensitive to movement. Needle aspiration of the toe confirms uric acid crystals, and he is treated with oral indomethacin (NSAID). One month later, he remains symptom free, and allopurinol is recommended for prevention of this condition. Which of the following is the most likely mechanism of action of allopurinol?
A 54-year-old man comes to the emergency room with severe pain in his right toe. He has had less severe episodes in the past, which he always treated with pain medications. The toe is red, inflamed, and exquisitely sensitive to movement. Needle aspiration of the toe confirms uric acid crystals, and he is treated with oral indomethacin (NSAID). One month later, he remains symptom free, and allopurinol is recommended for prevention of this condition. Which of the following is the most likely mechanism of action of allopurinol?
💡 Explanation
## **Core Concept**
The patient's presentation is suggestive of **gout**, a form of inflammatory arthritis characterized by sudden, severe attacks of pain, swelling, redness, and tenderness. Gout is caused by elevated levels of uric acid in the blood, which can form sharp, needle-like urate crystals in a joint or surrounding tissue, leading to pain, inflammation, and swelling. The management of gout involves treating acute attacks and preventing future episodes.
## **Why the Correct Answer is Right**
Allopurinol is a **xanthine oxidase inhibitor**. Xanthine oxidase is an enzyme responsible for the production of uric acid in the body. It catalyzes the conversion of hypoxanthine to xanthine and then xanthine to uric acid. By inhibiting xanthine oxidase, allopurinol reduces the production of uric acid, thereby lowering serum urate levels and preventing the formation of urate crystals. This mechanism is crucial for the long-term management and prevention of gout attacks.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe the mechanism of action of allopurinol. Without the specific details of the option, we can infer that any option not describing xanthine oxidase inhibition would be incorrect for allopurinol.
- **Option B:** Similarly, this option is incorrect if it does not align with the mechanism of xanthine oxidase inhibition.
- **Option D:** This option is also incorrect if it suggests a different mechanism of action not related to xanthine oxidase inhibition.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that allopurinol is used for the long-term management of gout to prevent future attacks by lowering uric acid levels. However, it is not used to treat acute gout attacks. In fact, initiating allopurinol during an acute gout attack can potentially worsen the condition. A common mnemonic to recall drugs that can induce or worsen gout is "LADIES," which stands for Loop diuretics, Aspirin (low dose), Diabetes drugs (some), Immunosuppressants, Ethanol, and Steroids (withdrawal).