**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 joint space. This condition is often associated with elderly individuals and can lead to joint pain and inflammation.

**Why the Correct Answer is Right**
The correct answer is CPPD crystals. These crystals are formed due to an imbalance in the metabolism of calcium and pyrophosphate within the joint fluid. The accumulation of CPPD crystals triggers an inflammatory response, leading to joint pain and swelling. The crystals are usually found within the synovial fluid of the affected joint, where they can be identified through arthrocentesis and crystal analysis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Monosodium urate (MSU) crystals are associated with gout, a different type of crystal arthritis. While both conditions can cause joint pain and inflammation, they have distinct underlying pathologies and are caused by different types of crystals.
* **Option B:** Hydroxyapatite crystals are associated with a rare form of arthritis known as hydroxyapatite deposition disease. However, this condition is distinct from pseudogout and is not caused by CPPD crystals.
* **Option C:** Calcium oxalate crystals are not typically associated with joint disease and are more commonly found in the context of kidney stones.

**Clinical Pearl / High-Yield Fact**
CPPD crystal deposition disease is often associated with other underlying conditions, such as hyperparathyroidism, hemochromatosis, and aging. Identifying these underlying conditions is crucial for effective management and prevention of pseudogout attacks.

**Correct Answer: D. CPPD crystals.**