Needle shaped crystals negatively birefringent on polarized microscopy is characteristic of which crystal associated ahropathy?
**Core Concept**
The question is testing the characteristic microscopic features of various crystal-associated arthropathies, specifically focusing on the shape and birefringence of crystals on polarized microscopy.
**Why the Correct Answer is Right**
Needle-shaped crystals that exhibit negative birefringence on polarized microscopy are characteristic of calcium pyrophosphate dihydrate (CPPD) crystals. This is because CPPD crystals have a distinct needle-like morphology and exhibit negative birefringence due to their unique crystal structure. Negative birefringence is a key feature that distinguishes CPPD crystals from other types of crystals, such as monosodium urate (MSU) crystals, which are typically negatively birefringent but have a different morphology.
**Why Each Wrong Option is Incorrect**
**Option A:** Hydroxyapatite crystals are typically seen in conditions like pseudogout, but they are not needle-shaped and exhibit positive birefringence, not negative.
**Option B:** Monosodium urate (MSU) crystals are associated with gout and are typically negatively birefringent, but they have a characteristic "dog-bone" or "flea-bitten" appearance on polarized microscopy.
**Option C:** Basic calcium phosphate crystals are associated with conditions like chondrocalcinosis, but they are typically seen as small, poorly defined particles rather than needle-shaped crystals.
**Clinical Pearl / High-Yield Fact**
When evaluating a patient with joint pain and swelling, remember that the presence of needle-shaped crystals with negative birefringence on polarized microscopy is highly suggestive of CPPD disease (pseudogout). This is a classic exam-trap, as many students mistakenly associate negative birefringence with MSU crystals.
**Correct Answer:** C. CPPD crystals