Match the following synovial fluid analysis findings to their most likely corresponding jo… — USMLE Step 2 CK

Match the following synovial fluid analysis findings to their most likely corresponding joint disorder.

Needle-shaped, strongly negatively birefringent crystals under polarized light microscopyGouty arthritis
Rhomboid-shaped, weakly positively birefringent crystals under polarized light microscopyCalcium pyrophosphate deposition disease
Opaque, purulent fluid with a leukocyte count of $>50,000/\mu\text{L}$ and $>90\%$ neutrophilsSeptic arthritis
Clear, viscous fluid with a leukocyte count of $<2,000/\mu\text{L}$ and $<25\%$ neutrophilsOsteoarthritis

Answer

Needle-shaped negative crystals correspond to gouty arthritis. Rhomboid-shaped positive crystals correspond to calcium pyrophosphate deposition disease. Opaque fluid with >50,000 leukocytes corresponds to septic arthritis. Clear fluid with <2,000 leukocytes corresponds to osteoarthritis.

The correct matches rely on standard synovial fluid diagnostic criteria. Noninflammatory fluid (leukocytes

<2,000/\mu\text{L}

) points to osteoarthritis. Purulent fluid with highly elevated leukocytes (

>50,000/\mu\text{L}

) and neutrophil dominance indicates septic arthritis and requires immediate intervention. Crystal arthropathies are differentiated by their unique microscopic profiles: gout presents with negatively birefringent needle-like urate crystals, whereas pseudogout presents with positively birefringent rhomboid calcium pyrophosphate crystals.

Step-by-Step Solution

Evaluate the synovial fluid leukocyte count and differential.

Identify whether the fluid is noninflammatory (

<2,000/\mu\text{L}

), inflammatory, or purulent/septic (

>50,000/\mu\text{L}

This broad categorization separates degenerative processes (osteoarthritis) from highly dangerous acute infections (septic arthritis).

Analyze the polarized light microscopy findings for crystals.

Differentiate between monosodium urate and calcium pyrophosphate crystals based on shape and birefringence.

Crystal shape and optical properties definitively distinguish between the two primary crystal arthropathies: gout and pseudogout.

Key Concept

Synovial Fluid Analysis in Rheumatology

Estimated Time:45s

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