Differential Diagnosis

• Single most likely diagnosis
  • Viral meningitis or aseptic meningitis: The patient's symptoms of headache, fever, and tenderness to palpation of the right posterior scalp and mastoid, along with a maculopapular rash, could suggest a viral etiology. The elevated CRP with normal WBC and lymphocytes also supports an inflammatory process that is not clearly bacterial. The response to Tylenol and Advil further points towards a viral or inflammatory cause rather than a bacterial one.
• Other Likely diagnoses
  • Temporal arteritis: Although less common in a 30-year-old, the tenderness to palpation of the scalp could suggest temporal arteritis, especially with the presence of headache and an elevated CRP. However, the absence of jaw claudication and visual symptoms makes this less likely.
  • Lyme disease: Given the maculopapular rash and the symptoms of headache and fever, Lyme disease could be considered, especially if the patient has been exposed to ticks. The rash's description does not specifically mention a "bull's-eye" pattern, but Lyme disease can present with various rash types.
  • Meningitis (bacterial or fungal): Although the patient is currently afebrile and the WBC is normal, bacterial or fungal meningitis cannot be ruled out without further investigation, such as a lumbar puncture, especially given the headache and scalp tenderness.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Subarachnoid hemorrhage: Although the patient does not have a decreased level of consciousness (ALOC) or severe headache at presentation, a subarachnoid hemorrhage could present with headache and might not always have an immediately altered mental status. The absence of severe headache and normal WBC makes this less likely, but it's a critical diagnosis not to miss.
  • Meningococcemia: This condition can present with fever, headache, and a rash, and it is critical to diagnose early due to its high mortality rate. The patient's vaccination status might reduce the likelihood, but it's not a diagnosis to overlook.
• Rare diagnoses
  • Syphilis (secondary): This could present with a rash and headache, among other symptoms. Given the patient's age and the presence of a maculopapular rash, syphilis is a possibility, although less likely without other specific symptoms or risk factors.
  • Autoimmune disorders (e.g., lupus): These can present with a wide range of symptoms including rash, fever, and headache. However, the constellation of symptoms and the specific findings would need to align more closely with an autoimmune disorder for it to be considered a primary diagnosis.