A 62-year-old man with a 15-year history of poorly controlled gout presents with multiple firm nodules on his ears, fingers, and elbows. On examination of the left ear helix, a nodule is noted to have ulcerated and is discharging a chalky white material. Microscopic examination of the material marked **D** in the diagram shows needle-shaped crystals that appear yellow when the compensator is parallel to the crystal axis under polarized light microscopy. Which of the following best describes the chemical composition and pathophysiology of this discharged material?

Explanation

## Why Monosodium urate monohydrate crystals is right The chalky white toothpaste-like material discharged from a chronic gouty tophus is pathognomonic for monosodium urate monohydrate crystals. The yellow birefringence when parallel to the compensator (negative birefringence) is the diagnostic hallmark under polarized light microscopy. Gouty tophi represent chronic deposits of these crystals that develop after 5–10+ years of untreated hyperuricemia, where serum uric acid exceeds the saturation point of 6.8 mg/dL. The helix of the ear is a classic site due to its cool temperature, low pH, and avascular cartilage—all conditions favoring crystal precipitation. This patient's 15-year history of poorly controlled gout and multiple tophi at characteristic sites (ear, fingers, olecranon) confirm chronic tophaceous gout with urate crystal deposition. (Harrison 21e, Ch 365) ## Why each distractor is wrong - **Calcium pyrophosphate dihydrate crystals**: These cause pseudogout and are rhomboid-shaped with positive birefringence (blue when parallel to compensator). They do not form chalky discharges from tophi and are associated with acute inflammatory arthritis, not chronic nodular deposits. - **Hydroxyapatite crystals**: These are associated with chronic renal disease, secondary hyperparathyroidism, and calcific periarthritis, not with chronic hyperuricemia or tophaceous gout. They do not discharge as chalky material from nodules on the ear. - **Cholesterol crystals**: Cholesterol deposits occur in xanthomas (lipid disorders) and atherosclerotic plaques, not in gouty tophi. They are not needle-shaped and do not show negative birefringence under polarized light. **High-Yield:** Needle-shaped, negatively birefringent crystals (yellow when parallel) = monosodium urate; rhomboid, positively birefringent crystals (blue when parallel) = calcium pyrophosphate dihydrate (pseudogout). [cite: Harrison 21e, Ch 365]