A microbiologist compares microbial communities from two urinary tract locations: the bladder and the urethral opening. The urethral opening consistently shows greater microbial variety. Which factor best explains this observation?

Higher nutrient concentration at the urethral opening stimulates spontaneous bacterial formation

Direct contact with external microbial sources near the urethral opening

Continuous bacterial production by epithelial cells in the urethra

Complete absence of antimicrobial peptides near the urethral opening