What is a key characteristic of hemimelia?

Hemimelia is a condition characterized by the partial absence of a limb. This means that, rather than the entire limb being missing, there are sections of it that are underdeveloped or absent, often affecting the distal parts of the limb. For example, in some cases of lower limb hemimelia, the foot or part of the leg may be underdeveloped or absent, while other structures, such as the knee or hip, may be present and functional.

In contrast, the other options suggest different conditions or situations. Complete absence of a limb would denote amelia, not hemimelia. The presence of additional digits pertains to polydactyly and is unrelated to limb absence. Retained proximal structure only suggests a specific absence of distal elements more commonly seen in cases of amelia or other limb development disorders, which does not accurately represent hemimelia.

Thus, the defining feature of hemimelia is its partial presence, aligning with the description of the condition.