Which type of capillary is characterized by small pores allowing some protein passage?

Fenestrated capillaries are specifically designed with small pores or "fenestrations" in their endothelial cell walls, which allow for greater permeability compared to other types of capillaries. This unique structure facilitates the passage of not only fluids but also small solutes and proteins. They are commonly found in tissues that engage in intensive exchange processes, such as the kidneys, intestines, and endocrine glands.

Continuous capillaries, in contrast, have a more solid endothelial lining with tight junctions, making them much more selective about what can pass through the vessel wall. This limits their permeability primarily to water and small molecules, while larger proteins are generally restricted.

Sinusoidal capillaries have larger gaps between the endothelial cells, allowing for the free passage of larger molecules and even entire cells. Their structure is advantageous for organs involved in filtration and exchange, such as the liver, spleen, and bone marrow, but they do not possess the small pores characteristic of fenestrated capillaries.

The option "All types of capillaries" does not accurately reflect the distinctions among the types since only fenestrated capillaries have the specific characteristic of small pores that allow protein passage selectively. Thus, the most accurate answer is fenestrated