MSU Crystal Identification — SCE Rheumatology MCQ
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Correct answer: E — Monosodium urate
The crystals are monosodium urate (MSU), making C correct. MSU crystals are typically slender and needle-shaped and show strong negative birefringence under compensated polarised light microscopy; in this clinical setting, their identification confirms gout. Calcium pyrophosphate dihydrate crystals are usually rhomboid or short rod-shaped and weakly positively birefringent. Basic calcium phosphate crystals are generally too small to be resolved by routine polarised light microscopy. Calcium oxalate crystals are typically bipyramidal or envelope-shaped and positively birefringent, while cholesterol crystals form larger plate-like structures with notched corners. Detection of MSU crystals does not, however, exclude concurrent septic arthritis if infection remains clinically suspected.
Reference: Berendsen D, et al. Crystal identification of synovial fluid aspiration by polarized light microscopy. Clinical Rheumatology. 2017;36:641–647. https://pubmed.ncbi.nlm.nih.gov/27837341/