**Core Concept**
The question is testing the knowledge of common hospital-acquired infections, which are infections that patients acquire during their hospital stay. These infections can be caused by bacteria, viruses, fungi, or other microorganisms that are present in the hospital environment.

**Why the Correct Answer is Right**
Urinary tract infections (UTIs) are the most common type of nosocomial infection, accounting for approximately 40% of all hospital-acquired infections. They are often caused by the introduction of bacteria into the urinary tract through catheters or other medical devices. The most common bacteria responsible for UTIs in hospitals are Escherichia coli (E. coli), Klebsiella pneumoniae, and Pseudomonas aeruginosa. These bacteria can thrive in hospital environments, particularly in areas with contaminated water or inadequate infection control practices.

**Why Each Wrong Option is Incorrect**
* **Option A:** Surgical site infections are a significant concern in hospitals, but they are not the most common type of nosocomial infection. They occur when bacteria are introduced into the surgical site during surgery.
* **Option B:** Bloodstream infections, such as sepsis, are serious and potentially life-threatening, but they are not the most common type of nosocomial infection.
* **Option C:** Pneumonia is a common hospital-acquired infection, but it is not the most common type of nosocomial infection.

**Clinical Pearl / High-Yield Fact**
The Centers for Disease Control and Prevention (CDC) estimate that approximately 1 in 31 hospital patients will acquire a hospital-acquired infection, resulting in significant morbidity, mortality, and healthcare costs.

**Correct Answer: C. Urinary tract infections (UTIs).**