You’re cruising through a Q-bank, you see fever + headache + altered mental status in late summer, and your brain whispers “viral encephalitis.” Then the answer choices hit: HSV-1, West Nile, rabies, St. Louis encephalitis, VZV… and suddenly everything feels plausible. This is exactly where points are won or lost on Step exams: not just knowing the right answer, but knowing why the wrong ones are wrong.
Tag: Microbiology > Virology
The Clinical Vignette (Classic Q-bank Style)
A 68-year-old man presents in August with fever, headache, confusion, and tremors. He lives in the Midwest and spends evenings outside. Exam shows nuchal rigidity and mild weakness. CSF: lymphocytic pleocytosis, mildly ↑ protein, normal glucose. MRI is nonspecific. A few days into admission, he develops flaccid weakness.
Question: Which pathogen is the most likely cause?
Step Approach: Identify the Syndrome First
This vignette screams:
- Seasonal neuroinvasive disease (late summer/early fall)
- Mosquito exposure
- Encephalitis ± meningitis
- Movement disorder features (tremor)
- Acute flaccid paralysis (anterior horn cell involvement)
That combo is a board-style fingerprint for:
✅ Correct Answer: West Nile virus
Why West Nile Fits Best
West Nile virus (WNV) is a mosquito-borne flavivirus and the most common cause of arboviral neuroinvasive disease in the US.
High-yield features:
- Transmission: Mosquito (often Culex), birds are the main reservoir
- Season: Late summer to early fall
- Neuroinvasive disease: meningitis, encephalitis, or acute flaccid myelitis-like illness
- Clue symptoms:
- Tremor, myoclonus, parkinsonian features
- Acute flaccid paralysis (anterior horn cell damage → polio-like)
- CSF: lymphocytic pleocytosis, normal glucose (typical viral pattern)
- Diagnosis (boards + real life): WNV IgM in CSF (or serum IgM), PCR less sensitive later
Quick Memory Hook
West Nile = “West in late summer” + mosquitoes + neuroinvasive tremor/parkinsonism + flaccid paralysis.
Arboviral Encephalitis: The Core Framework (Testable)
What counts as “arboviral encephalitis” on exams?
Usually mosquito-borne RNA viruses causing encephalitis, commonly:
- Flaviviruses: West Nile, St. Louis encephalitis, Japanese encephalitis, dengue (less encephalitis-focused)
- Alphaviruses (Togaviridae): Eastern/Western/Venezuelan equine encephalitis
General arboviral pattern
| Feature | Arboviral encephalitis (general) |
|---|---|
| Season | Summer/early fall |
| Vector | Mosquito (classically) |
| CSF | Viral pattern: ↑ lymphocytes, mildly ↑ protein, normal glucose |
| Imaging | Often nonspecific; not the HSV temporal lobe slam dunk |
| Treatment | Supportive (no acyclovir benefit unless HSV suspected) |
Now the Money: Why Each Distractor Is Wrong (or Less Right)
Distractor 1: HSV-1
Why it tempts you: It’s the most tested cause of sporadic fatal encephalitis, and empiric acyclovir is a reflex in real life.
Why it’s wrong here:
- HSV-1 encephalitis is not seasonal and not tied to mosquito exposure.
- Classic clue is temporal lobe involvement → personality change, aphasia, focal seizures.
- MRI often shows temporal lobe hyperintensity; CSF may show RBCs (hemorrhagic encephalitis).
- Diagnosis: PCR of CSF.
Board takeaway:
If you see temporal lobes + focal neuro + RBCs in CSF, think HSV-1. If you see late summer + tremor + flaccid paralysis, think West Nile.
Distractor 2: Rabies virus
Why it tempts you: Encephalitis + scary neuro symptoms.
Why it’s wrong here:
- Exposure is typically animal bite (bats, raccoons, dogs in endemic areas).
- Pathognomonic clinical clue: hydrophobia (painful pharyngeal spasms), aerophobia, agitation.
- Incubation can be weeks to months (not a quick “got bit last weekend” story).
- Not a mosquito-borne seasonal illness.
High-yield fact: Rabies travels via retrograde axonal transport to CNS; Negri bodies in hippocampus/Purkinje cells.
Distractor 3: St. Louis encephalitis virus
Why it tempts you: Also a mosquito-borne flavivirus causing encephalitis in late summer.
Why it’s less right than West Nile (what the vignette is pushing):
- St. Louis encephalitis can cause fever, headache, AMS, and occurs in similar seasons.
- But the vignette’s “bonus clue” is acute flaccid paralysis, which is far more classically tied to West Nile (anterior horn cell involvement).
- WNV is also the most common arboviral neuroinvasive disease in the US.
Test strategy:
If the question includes polio-like flaccid paralysis, WNV jumps to the top.
Distractor 4: Varicella-zoster virus (VZV)
Why it tempts you: Neurotropic herpesvirus; can cause meningoencephalitis, vasculopathy, and strokes.
Why it’s wrong here:
- Often associated with rash (though can occur without rash).
- VZV is a classic cause of vasculopathy → multifocal strokes, especially in immunocompromised or older adults.
- Not specifically seasonal or linked to mosquito exposure.
- Diagnosis: CSF PCR or intrathecal antibody.
When to pick VZV:
Encephalitis with stroke-like deficits, immunocompromise, dermatomal pain/rash, or imaging suggesting vasculitis.
Distractor 5: Eastern equine encephalitis (EEE)
Why it tempts you: It’s a classic arboviral encephalitis.
Why it’s less right (depending on region/clues):
- EEE is rare but severe (high mortality) and more associated with Atlantic/Gulf Coast areas, swampy habitats.
- Presents with abrupt severe encephalitis; can have seizures, coma.
- The vignette’s “most common in US + flaccid paralysis” points harder to WNV.
High-yield micro pearl:
EEE is an alphavirus (Togaviridae); tends to be more lethal than WNV.
High-Yield “Answer Choice Sorting” Table
| Virus | Vector/Exposure | Key Clues | Diagnostics | Pearl |
|---|---|---|---|---|
| West Nile (Flavivirus) | Mosquito, late summer | Tremor/parkinsonism, acute flaccid paralysis, encephalitis | CSF IgM | Most common US arboviral neuroinvasive disease |
| HSV-1 | Reactivation; not seasonal | Temporal lobe, focal seizures, aphasia, CSF RBCs | CSF PCR | Treat with acyclovir |
| Rabies | Animal bite (bat/raccoon) | Hydrophobia, agitation → coma | PCR/antibodies; Negri bodies | Retrograde axonal transport |
| St. Louis encephalitis (Flavivirus) | Mosquito, summer | Encephalitis, older adults | Serology | Similar to WNV but less “flaccid paralysis” vibe |
| VZV | Reactivation | Rash ± encephalitis, vasculopathy/strokes | CSF PCR | Think vasculitis, immunocompromise |
| EEE (Alphavirus) | Mosquito, swampy areas | Severe encephalitis, high mortality | Serology | Rare but deadly |
The “Don’t Miss” USMLE Moves
1) CSF patterns still matter
- Viral: lymphocytes, normal glucose, mildly ↑ protein
- HSV can add RBCs due to hemorrhagic necrosis (not required, but classic).
2) Empiric management is a favorite Step twist
In real life, if encephalitis is suspected:
- Start acyclovir empirically (until HSV is excluded), because HSV is treatable and devastating.
- Arboviruses (including WNV) → supportive care.
3) “Acute flaccid paralysis” narrows fast
Think anterior horn cell involvement:
- West Nile (big one in US)
- Polio/enteroviruses (depending on vignette context)
Take-Home Summary (What You Should Remember on Test Day)
- West Nile virus: late summer mosquito exposure + encephalitis + tremor/parkinsonism ± acute flaccid paralysis → diagnose with CSF IgM.
- HSV-1: temporal lobe encephalitis, focal seizures/aphasia, CSF PCR, treat with acyclovir.
- Distractors often differ by seasonality, exposure history, and one signature clinical clue.