Most of the cases of Listeria are due to serovar –

**Core Concept**
Listeriosis is a foodborne illness caused by the bacterium Listeria monocytogenes. This pathogen is a type of gram-positive, rod-shaped bacterium that can cause a range of diseases, from mild gastroenteritis to severe infections such as meningitis and septicemia.

**Why the Correct Answer is Right**
Listeria monocytogenes is the most virulent serovar of the bacterium Listeria, responsible for the majority of human infections. This serovar has a high affinity for the gastrointestinal tract and can invade the epithelial cells, leading to the production of virulence factors such as phospholipase C and listeriolysin O. These virulence factors help the bacterium evade the host's immune response and cause disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because serovar 4b is another virulent serovar, but it is not the most common cause of listeriosis.
* **Option B:** This option is incorrect because serovar 1/2a is a common serovar found in animals, but it is not typically associated with human disease.
* **Option C:** This option is incorrect because serovar 4a is a less virulent serovar and is not commonly associated with human infections.

**Clinical Pearl / High-Yield Fact**
Listeria monocytogenes is a zoonotic pathogen, meaning it can be transmitted from animals to humans. Pregnant women, immunocompromised individuals, and older adults are at increased risk of developing listeriosis.

**Correct Answer:** .