Final answer:
The most commonly occurring nosocomial infection is a Urinary tract infection (UTI), often associated with catheter use in hospital settings.
Step-by-step explanation:
The most commonly occurring nosocomial infection is a Urinary tract infection (UTI). Nosocomial, or hospital-acquired infections, often occur when medical interventions such as surgeries provide an opportunity for pathogens to enter the body. While pneumonia is a significant cause of sickness among hospitalized individuals, particularly ventilator-associated pneumonia, UTIs are more frequently reported due to factors like catheterization. The bacteria often responsible for UTIs include E. coli from the patient's own flora, introduced into the urinary tract. Roberta's case, where UTI symptoms are present after surgery, is a classic example of a nosocomial UTI, prompting her physician to diagnose her condition as such.