Final answer:
The cell with a U-shaped nucleus and light blue cytoplasm observed under the microscope on a blood slide is a monocyte, which is a type of leukocyte with a large size and distinct U-shaped nucleus.
Step-by-step explanation:
After observing a blood slide under the microscope which shows a large cell with a U-shaped nucleus and light blue cytoplasm. The cell you have identified is most likely a monocyte. Monocytes are a type of leukocyte (white blood cell) with a distinct U-shaped or kidney-shaped nucleus and abundant cytoplasm that can appear light blue to greyish when stained with standard histological stains like Wright's. They tend to be larger than other leukocytes, typically around 12-20 µm in diameter.
For comparison, neutrophils, which have a multi-lobed nucleus and lighter staining granules in their cytoplasm, and lymphocytes, characterized by their large, dark-staining nuclei and very little cytoplasm, differ significantly in their appearances from monocytes. Basophils have large dark staining granules that may obscure their two-lobed nucleus, which is not consistent with the U-shaped nucleus description given.