What condition describes a fracture but it is not displaced?

The correct answer relates to the definition of spondylolysis, which is characterized by a defect or fracture in the pars interarticularis of a vertebra. Importantly, this condition often does not involve displacement of the vertebrae, meaning the alignment remains intact despite the fracture.

Spondylolysis typically occurs due to repetitive stress, often seen in athletes, and may not show signs of displacement because the fracture itself lies within the bony structure and does not lead to a shift in position of the vertebrae.

In contrast, other conditions presented in the options involve displacement or misalignment. For instance, spondylolisthesis involves one vertebra slipping forward over another, which is clearly a displaced condition. Similarly, retrolisthesis refers to one vertebra sliding backward in relation to another, indicating a different type of alignment issue. Spondylosis, on the other hand, relates to degeneration of the spinal column and does not describe a fracture at all.

Thus, spondylolysis fits the criteria as a non-displaced fracture of the vertebra, making it the appropriate answer.