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: Surgery
Commonest organism giving rise to urinary tract infection - 
Select and explain. 
A. E. coli, B. Proteus, C. Staphylococcus, D. Streptococcus
A: E. coli
Answer: A: E. coli 
Explanation: • Most UTIs are caused by a single bacterial species.
• At least 80% of the uncomplicated cystitis and pyelonephritis are due to E. coli, with most of pathogenic strains belonging to the O serogroups.
• Other less common uropathogens include Klebsiella, Proteus, and Enterobacter spp. and enterococci.
• In hospital acquired UTIs, a wider variety of causative organisms is found, including Pseudomonas and Staphylococcus spp.
• UTIs caused by S. aureus often result from hematogenous dissemination.
• Group B beta-hemolytic streptococci can cause UTIs in pregnant women.
• In children, Klebsiella and Enterobacter spp. are common causes of UTI.
• Anaerobic bacteria, lactobacilli, corynebacteria, streptococci (not including enterococci) and S. epidermidis are found in normal periurethral flora. They do not commonly cause UTIs in healthy individuals and are considered common urinary contaminants.