**Question:** Commonest cause of nosocomial infection is

A. Methicillin-resistant Staphylococcus aureus (MRSA)
B. Pseudomonas aeruginosa
C. Escherichia coli (E. coli)
D. Candida albicans

**Correct Answer:** A. Methicillin-resistant Staphylococcus aureus (MRSA)

**Core Concept:** Nosocomial infections are infections that occur in patients while they are receiving medical care in a healthcare setting. These infections are a major concern due to the increased morbidity, mortality, and healthcare costs associated with them.

**Why the Correct Answer is Right:** MRSA (methicillin-resistant Staphylococcus aureus) is the commonest cause of nosocomial infection because it has developed resistance to multiple antibiotics, including methicillin, making it difficult to treat. Staphylococcus aureus is a common skin and nasal flora bacterium that can cause infections when it invades the body through breaks in the skin or other routes. The methicillin resistance in MRSA is due to the presence of the mecA gene, which encodes for a penicillin-binding protein (PBP2a) that is less susceptible to beta-lactam antibiotics, including methicillin.

**Why Each Wrong Option is Incorrect:**

B. Pseudomonas aeruginosa is a common cause of nosocomial infections, but it is less common than MRSA.
C. Escherichia coli (E. coli) is a common cause of community-acquired infections, not nosocomial infections.
D. Candida albicans is a common cause of fungal infections, particularly in immunocompromised patients, but it is not the primary cause of nosocomial infections.

**Clinical Pearl:** Nosocomial infections are a significant healthcare challenge, and preventing them requires rigorous hand hygiene, isolation precautions, and antibiotic stewardship programs. Early recognition and treatment of suspected infections are crucial to reduce morbidity, mortality, and healthcare costs associated with these infections.