Final answer:
While spontaneous pneumothorax involves lung collapse and connective tissue, non-union of fractures is related to metabolic issues like vitamin D deficiency or calcium insufficiency which directly affect bone growth and repair.
Step-by-step explanation:
The most common underlying metabolic abnormality in non-union, which refers to the failure of a fractured bone to heal properly, is often related to insufficient bone growth or repair mechanisms. While the question seems to be more about orthopedic issues, it mentions spontaneous pneumothorax, which is a collapsed lung event thought to be associated with irregularities in connective tissue. However, if we focus on non-union of bones, factors like inadequate blood supply, mechanical instability, and infection can play a significant role, as well as metabolic issues such as vitamin D deficiency or calcium insufficiency impacting bone remodeling processes.