**Core Concept**
Amenorrhea and altered mental status in a patient with fever, headache, and vomiting, particularly with a travel history, can suggest an infection that affects the central nervous system (CNS). The presence of polymorphonuclear neutrophils (PMNs) in the cerebrospinal fluid (CSF) indicates a bacterial infection.

**Why the Correct Answer is Right**
The high fever, headache, and confusion in this patient, combined with the presence of PMNs in the CSF, suggest a bacterial meningitis. Given the patient's travel history to Southern Minnesota, where Lyme disease is prevalent, this could be a case of Lyme meningitis. Lyme disease is caused by the bacterium *Borrelia burgdorferi*, which is transmitted by the bite of an infected black-legged tick. The infection can affect the CNS, causing symptoms such as meningitis, encephalitis, and radiculopathy.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option would be incorrect because the patient's symptoms and the presence of PMNs in the CSF do not suggest a viral infection. Viral meningitis typically presents with lymphocytic pleocytosis in the CSF.
* **Option B:** This option would be incorrect because the patient's symptoms and the presence of PMNs in the CSF do not suggest a fungal infection. Fungal meningitis typically presents with lymphocytic pleocytosis in the CSF and is often seen in immunocompromised patients.
* **Option C:** This option would be incorrect because the patient's symptoms and the presence of PMNs in the CSF do not suggest a parasitic infection. Parasitic meningitis is rare and typically presents with lymphocytic pleocytosis in the CSF.

**Clinical Pearl / High-Yield Fact**
Lyme disease is the most common tick-borne illness in the United States, and its incidence is increasing. It is essential to consider Lyme disease in patients with fever, headache, and altered mental status, particularly those with a travel history to areas where the disease is prevalent.

**Correct Answer:** C.