**Question:** The most frequent source of infection with Listeria monocytogenes is through which of the following?

A. Dairy products
B. Undercooked meat
C. Environmental sources
D. Unpasteurized juice

**Core Concept:**
Listeria monocytogenes is a Gram-positive, facultatively anaerobic bacterium that can cause listeriosis, an infection typically affecting immunocompromised individuals, newborns, and pregnant women. Foodborne transmission is the most common route of infection.

**Why the Correct Answer is Right:**
The correct answer is D. Unpasteurized juice, as it is one of the primary sources of Listeria monocytogenes contamination. Unpasteurized juice is not heated to the high temperatures required to kill the bacteria, allowing it to survive and potentially contaminate the juice. The consumption of contaminated unpasteurized juice can lead to the bacterium entering the human gastrointestinal tract and causing listeriosis.

**Why Each Wrong Option is Incorrect:**
A. Dairy products (e.g., cheese, yogurt) are also a common source of Listeria monocytogenes infection, but the correct answer addresses the most common source of contamination, which is unpasteurized juice.
B. Undercooked meat is another source of Listeria monocytogenes infection, but it is less common compared to unpasteurized juice.
C. Environmental sources are indirect sources, such as soil, water, and surfaces, which are not the primary entry point for the bacterium into the human body.

**Clinical Pearl:**
Understanding the most common source of Listeria monocytogenes infection is crucial for healthcare professionals, particularly for identifying potential risk factors and preventing outbreaks. Promptly discard and avoid consuming unpasteurized juice to minimize the risk of listeriosis, especially for vulnerable populations like the previously mentioned groups.

**Correct Answer:** D. Unpasteurized juice