**Core Concept**
Pseudogout, also known as calcium pyrophosphate deposition disease (CPPD), is a type of arthritis characterized by the deposition of calcium pyrophosphate dihydrate (CPPD) crystals within the joint space. The pathognomonic finding in pseudogout refers to the specific crystal type that is typically identified in joint fluid analysis.

**Why the Correct Answer is Right**
The correct answer is related to the identification of CPPD crystals in joint fluid analysis. These crystals are typically needle-shaped and have a characteristic "Maltese cross" appearance under polarized light microscopy. The presence of CPPD crystals is a diagnostic hallmark of pseudogout, distinguishing it from other forms of arthritis such as gout, which is caused by the deposition of monosodium urate crystals.

**Why Each Wrong Option is Incorrect**
**Option A:** Monosodium urate crystals are the pathognomonic finding in gout, not pseudogout.
**Option B:** Hydroxyapatite crystals are associated with another type of arthritis, known as hydroxyapatite deposition disease.
**Option C:** Basic calcium phosphate crystals are associated with another type of arthritis, known as basic calcium phosphate deposition disease.

**Clinical Pearl / High-Yield Fact**
When analyzing joint fluid for crystals, it's essential to use polarized light microscopy to visualize the characteristic appearance of CPPD crystals, which can help distinguish pseudogout from other forms of arthritis.

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