Clinical virology diagnostics: for which central nervous system infections is fluorescence microscopy useful? Select the best option summarizing where immunofluorescence or fluorescence microscopy can aid in diagnosis.

Introduction / Context:
Fluorescence microscopy, especially immunofluorescence (IF), is a rapid antigen-detection technique in clinical virology. This question checks whether you recognize its applicability to several central nervous system infections where speed can be lifesaving or supportive for diagnosis.

Given Data / Assumptions:

• IF detects viral antigens in tissue imprints or clinical specimens using labeled antibodies.
• Rabies diagnosis classically uses direct fluorescent antibody testing on brain tissue.
• Herpes simplex virus (HSV) antigens in lesions or cerebrospinal fluid cells can be detected in some laboratories (though PCR is now standard).
• SSPE is a chronic, measles-associated condition; IF may demonstrate measles antigens in brain tissue in specialized settings.

Concept / Approach:
While molecular assays (PCR) dominate modern diagnostics, fluorescence microscopy remains accepted or historically important in all three entities listed. It is definitive for rabies, supportive for HSV, and demonstrable in SSPE neuropathology. Therefore, the most inclusive correct answer is “All of these.”

Step-by-Step Solution:
Link rabies to direct fluorescent antibody (DFA) testing of brain smears.Acknowledge HSV antigen detection by IF in certain clinical specimens.Recognize measles antigen demonstration by IF in SSPE brain tissue sections.Choose the option encompassing all three uses.

Verification / Alternative check:
Public health references cite DFA as a frontline test for rabies. Hospital virology labs historically employed IF panels for HSV antigens; neuropathology texts note IF detection of measles antigens in SSPE.

Why Other Options Are Wrong:
Each single-condition option is incomplete, as the method applies across all listed diseases. “None of these” contradicts well-established rabies DFA testing.

Common Pitfalls:
Assuming PCR has entirely replaced antigen detection. PCR is now common, but the question focuses on usefulness of fluorescence microscopy, which remains valid in indicated scenarios.

Final Answer:
All of these.