Which urine artifact appears as spheres with a cell wall and often shows concentric circles?

The correct choice is pollen grains, which appear as distinctive spheres that have a defined cell wall and commonly exhibit concentric circles in their structure. This characteristic morphology is due to the layered structure of the pollen grains, which can be visualized under a microscope, distinguishing them from other artifacts that may be found in urine.

Pollen grains are biologically derived from plants and are notably larger than many other urinary sediment components, contributing to their unique appearance. Their presence in urine can be indicative of environmental exposure, particularly related to seasonal allergies.

In contrast, air bubbles are typically not solid structures and will not have a defined wall or concentric circles; they appear as clear areas within the urine sample. Starch has a different morphology, presenting as granules, and does not easily form the spherical shapes with cell walls and concentric patterns seen in pollen. Fibers may be elongated and can vary in appearance but lack the spherical and concentric characteristics associated with pollen grains, making them distinct from the correct answer.