What specifically denotes the absence of the ulna?

The absence of the ulna is specifically denoted by paraxial hemimelia. This condition refers to the congenital absence of bones located in the axial skeleton, which includes both the radius and ulna in the context of the upper limb. Paraxial hemimelia indicates a defect along the axis of the limb where structures like the ulna may be absent or severely hypoplastic.

In contrast, specific terms such as pre-axial and post-axial hemimelia refer to conditions related to either the radial side (thumb side) or ulnar side (little finger side) of the limb, respectively. Pre-axial radial hemimelia specifically describes the absence of the radius, which affects the radial side, while post-axial conditions focus on the ulnar side. Since the question strictly pertains to the absence of the ulna, paraxial hemimelia accurately encompasses scenarios where this type of limb deficiency occurs without confusion regarding the more specific aspects of radial involvement.

Thus, in summarizing paraxial hemimelia, it provides a clear contextual understanding of the absence of the ulna as a part of broader limb development anomalies.