**Question:** A 30-year old patient presented with features of acute meningoencephalitis in the casualty. His CSF on wet mount microscopy revealed motile unicellular microorganisms. The most likely organism is -

A. Cryptococcus neoformans
B. Neisseria meningitidis
C. Streptococcus pneumoniae
D. Listeria monocytogenes

**Correct Answer:** D. Listeria monocytogenes

**Core Concept:** Meningoencephalitis is an inflammation of the meninges (protective membranes) surrounding the brain and spinal cord, which can be caused by various pathogens. In this scenario, the patient exhibits symptoms and laboratory findings consistent with meningoencephalitis due to motile unicellular microorganisms found in the cerebrospinal fluid (CSF) on wet mount microscopy.

**Why the Correct Answer is Right:**

Listeria monocytogenes is a Gram-positive bacterium that can cause meningitis and encephalitis, particularly in immunocompromised individuals or when the infection is disseminated from an abdominal source. The organism is typically found in contaminated dairy products, soil, and water. The correct answer (D) refers to Listeria monocytogenes because:

1. It is a common cause of meningoencephalitis, particularly in immunocompromised patients and when the infection has spread from an abdominal source.
2. The wet mount microscopy result of motile unicellular microorganisms in the CSF is consistent with Listeria monocytogenes infection, as other options do not have this characteristic feature.

**Why Each Wrong Option is Incorrect:**

A. Cryptococcus neoformans: This fungus is responsible for cryptococcal meningitis, which is non-motile and occurs primarily in immunocompromised patients (e.g., HIV infection) or in individuals with pulmonary cryptococcosis.

B. Neisseria meningitidis: This bacterium causes meningitis, but is typically non-motile and typically affects immunocompetent individuals.

C. Streptococcus pneumoniae: This bacterium causes bacterial meningitis, but it is typically non-motile and affects immunocompetent individuals.

**Clinical Pearls:**

Listeria monocytogenes meningoencephalitis is a rare but important infection to consider, particularly in patients with compromised immunity or with a history of consuming contaminated food or water. Early diagnosis and appropriate antibiotic treatment can lead to a favorable prognosis, despite the severe nature of the infection.