This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
The most probable cause of crowding in lower anterior region is: 
Select and explain. 
A. Prolonged retention of lower primary incisors, B. Premature exfoliation of tower primary incisors, C. Presence of supernumerary teeth, D. Tooth-size-arch length discrepancy
D: Tooth-size-arch length discrepancy
Answer: D: Tooth-size-arch length discrepancy 
Explanation: None