**Question:** A 63-year-old man develops pain and swelling in his knee. It appears warm, red, and swollen with decreased range of movement. A diagnostic aspiration is performed. Which of the following will most likely distinguish pseudogout from gout?

A. Calcium pyrophosphate dehydrate crystals
B. Uric acid crystals
C. Both calcium pyrophosphate dehydrate and uric acid crystals
D. Patient's age and joint involvement

**Correct Answer:** C. Both calcium pyrophosphate dehydrate and uric acid crystals

**Core Concept:** Pseudogout and gout are both types of inflammatory arthritis caused by the deposition of crystals in the joint. However, they differ in their crystal composition and clinical presentation.

**Why the Correct Answer is Right:** Both pseudogout and gout involve the deposition of crystals in the joint, leading to inflammation and pain. However, the crystals involved are different.

Pseudogout is caused by calcium pyrophosphate dehydrate (CPPD) crystals, while gout is caused by uric acid crystals. Diagnosing between the two relies on identifying the specific crystal type present in the joint fluid aspirate.

**Why Each Wrong Option is Incorrect:**

A. Calcium pyrophosphate dehydrate crystals (CPPD) are the key component in pseudogout, not gout. Thus, identifying CPPD crystals would support a diagnosis of pseudogout but not gout.

B. Uric acid crystals are the primary component in gout, not pseudogout. Identifying uric acid crystals would support a diagnosis of gout but not pseudogout.

D. Patient's age and joint involvement do not directly differentiate between pseudogout and gout. The presence of specific crystals is crucial for the correct diagnosis.

**Clinical Pearl:** The presence of both calcium pyrophosphate dehydrate and uric acid crystals in the joint fluid aspirate will help differentiate between pseudogout and gout. This is important for appropriate treatment and prognosis. The identification of these crystals is essential in guiding the correct diagnosis and subsequent management plans.