A 69-year-old man has had mild arthritis involving many joints for several years. Over 1 or 2 days, he develops severe pain and swelling of his knee. His x-ray reveals calcifications in his articular cartilage. Which of the following is the most likely diagnosis?
**Core Concept**
Pseudogout, also known as calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, is a type of arthritis characterized by the accumulation of calcium pyrophosphate dihydrate crystals within the articular cartilage, leading to acute inflammatory joint pain and swelling.
**Why the Correct Answer is Right**
Pseudogout typically occurs in older adults with pre-existing joint degeneration. The sudden onset of severe joint pain and swelling, often accompanied by calcifications in the articular cartilage on radiographs, is a hallmark of this condition. The crystals in pseudogout are made of calcium pyrophosphate dihydrate, which precipitate in the joint space due to various metabolic and hormonal abnormalities, such as hyperparathyroidism or hypomagnesemia.
**Why Each Wrong Option is Incorrect**
**Option A:** Acute gout is caused by the deposition of monosodium urate crystals in the joint, typically affecting the base of the big toe or other weight-bearing joints. While both conditions present with acute joint inflammation, the underlying pathophysiology and crystal composition differ.
**Option B:** Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by symmetric polyarthritis, often involving the hands and feet. Although RA can cause acute joint inflammation, the presence of CPPD crystals and calcifications in the articular cartilage is not characteristic of this condition.
**Option D:** Infectious arthritis, caused by bacterial or fungal pathogens, typically presents with fever, chills, and a more rapid onset of joint inflammation. While both conditions can cause severe joint pain, the presence of CPPD crystals and calcifications in the articular cartilage is not a feature of infectious arthritis.
**Clinical Pearl / High-Yield Fact**
Pseudogout often presents with acute joint pain and swelling, and radiographs may reveal calcifications in the articular cartilage. The condition is more common in older adults with pre-existing joint degeneration, and its diagnosis is confirmed by the presence of CPPD crystals in the joint fluid.
**✓ Correct Answer: C. pseudogout**