In urine microscopy, large numbers of mononuclear cells usually suggest which condition?

Mononuclear cells are primarily composed of lymphocytes and monocytes, which play significant roles in the immune response. In urine microscopy, the presence of a large number of these cells is often indicative of an inflammatory response related to the kidney interstitium.

Interstitial nephritis, characterized by inflammation of the kidney interstitium, often presents with a noticeable increase in mononuclear cells in the urine. This condition can result from various factors such as medications, infections, or autoimmune diseases, leading to an infiltration of these immune cells as they respond to injury or infection.

Chronic infection might also show some mononuclear cells but is typically more associated with greater numbers of other types of inflammatory cells. Dehydration usually has a different effect on urine cell composition, often leading to concentrated urine with fewer cells. Acute infections generally produce a surge in neutrophils rather than mononuclear cells, as neutrophils are the primary responders to bacterial infections.

Thus, the significant presence of mononuclear cells in urine is most representative of interstitial nephritis, making it the condition most associated with this finding.