Microscopic examination of cerebrospinal fluid reveals gram-positive rods. Which of the following organisms is most likely responsible?

• A. Listeria monocytogenes
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Neisseria meningitidis

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the significance of the microscopic examination result. Gram-positive rods indicate bacteria that retain the crystal violet stain and appear rod-shaped under the microscope.
2. Step 2: Recall the common bacterial causes of meningitis and their Gram stain characteristics. For example, Streptococcus pneumoniae is a Gram-positive coccus, Haemophilus influenzae is a Gram-negative coccobacillus, and Neisseria meningitidis is a Gram-negative diplococcus.
3. Step 3: Identify which organism among the options is a Gram-positive rod. Listeria monocytogenes is known to be a Gram-positive rod-shaped bacterium.
4. Step 4: Correlate the morphology with the clinical context. Since the cerebrospinal fluid shows Gram-positive rods, Listeria monocytogenes is the most likely causative agent.
5. Step 5: Conclude that the presence of Gram-positive rods in cerebrospinal fluid points towards Listeria monocytogenes as the responsible organism.