Differential Diagnosis

The patient's presentation of a maculopapular rash, fever, bilateral calf pain, and vital signs indicating hypotension and tachycardia suggests an infectious etiology. Given the location in Arizona, certain diagnoses are more likely due to the regional prevalence of specific pathogens.

• Single most likely diagnosis

  • Rocky Mountain Spotted Fever (RMSF): This is a tick-borne illness caused by Rickettsia rickettsii, which is prevalent in the southwestern United States, including Arizona. The rash typically starts on the wrists and ankles and spreads centrally, and the patient's symptoms of fever, rash, and severe illness are consistent with RMSF. The treatment of choice for RMSF is Doxycycline.

• Other Likely diagnoses

  • Lyme Disease: Although less common in Arizona compared to other parts of the United States, Lyme disease can cause a rash (erythema migrans) and systemic symptoms like fever. However, the rash in Lyme disease usually does not start on the wrists and ankles and spread centrally.
  • Southern Tick-Associated Rash Illness (STARI): This condition, caused by the lone star tick, presents with a rash similar to erythema migrans but is less likely to be associated with systemic symptoms like high fever and hypotension.
  • Ehrlichiosis/Anaplasmosis: These are tick-borne diseases that can cause fever, rash, and systemic illness. They are less common than RMSF but should be considered in the differential diagnosis.

• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)

  • Meningococcemia: This is a severe, life-threatening infection caused by Neisseria meningitidis. It can present with a rash, fever, and shock, similar to the patient's symptoms. Although less likely, it is critical to consider due to its high mortality rate if not promptly treated with antibiotics like Ceftriaxone.
  • Streptococcal Toxic Shock Syndrome: This condition can cause a rash, fever, and severe systemic illness, including hypotension and organ failure. It requires immediate recognition and treatment with antibiotics like Clindamycin.

• Rare diagnoses

  • Typhus: Both murine and scrub typhus can cause fever, rash, and systemic illness. However, they are less common in the United States and would be considered rare diagnoses.
  • Leptospirosis: This bacterial infection can cause a wide range of symptoms, including fever, rash, and systemic illness. It is more commonly associated with exposure to contaminated water and is rare in this clinical context.