<prompt> Which is most likely source of RBC cast in Urinalysis?UreterKidneyBladderUrethra  <response>: The RBCs of glomerular origin are often dysmorphic when examined by phase-contrast microscopy. Irregular shapes of RBCs may also result from pH and osmolarity changes produced along the distal nephron. Hematuria with dysmorphic RBCs, RBC casts, and protein excretion 500 mgd is viually diagnostic of glomerulonephritis. RBC casts form as RBCs that enter the tubule fluid become trapped in a cylindrical mold of gelled Tamm-Horsfall protein. So the source of RBC Cast is kidney. Ref: Harrisons Internal Medicine, 18th Edition, Page 339 