Differential Diagnosis for a 46-year-old Female with Febrile Illness and Full Body Blanching Red Rash

• Single Most Likely Diagnosis
  • Viral Exanthem (e.g., measles, rubella, or roseola): The presentation of a febrile illness followed by a full body blanching red rash is classic for viral exanthems. These conditions are common and typically self-limiting, making them a likely cause for the symptoms described.
• Other Likely Diagnoses
  • Scarlet Fever: Caused by Group A beta-hemolytic streptococci, it presents with a rash that appears after a febrile illness, often accompanied by a sore throat. The rash of scarlet fever is typically more punctate and may desquamate.
  • Toxic Shock Syndrome (TSS): Although more severe and less common, TSS can present with fever, rash, and systemic symptoms. It's often associated with bacterial infections, particularly Staphylococcus aureus or Streptococcus pyogenes.
  • Kawasaki Disease: An acute febrile illness of childhood but can occur in adults, characterized by fever, rash, mucous membrane changes, and lymphadenopathy. The rash can be polymorphous, including a blanching red rash.
• Do Not Miss Diagnoses
  • Meningococcemia: A severe, life-threatening infection caused by Neisseria meningitidis, which can present with fever, rash, and systemic symptoms. The rash can be petechial or purpuric but may initially appear as a blanching red rash.
  • Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, transmitted by ticks. It presents with fever, headache, and a characteristic rash that usually appears 2-5 days after the onset of fever, starting as a blanching red rash.
  • Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): Severe skin and mucous membrane disorders, usually drug-induced, which can present with fever, rash, and systemic symptoms. The rash can be blanching and progresses to necrosis.
• Rare Diagnoses
  • Erythema Infectiosum (Fifth Disease): Caused by Parvovirus B19, it typically presents with a "slapped cheek" appearance followed by a reticular rash on the body. While common in children, it can occur in adults.
  • Secondary Syphilis: Can present with a variety of rashes, including a diffuse, maculopapular rash that may appear after a febrile illness. The rash of secondary syphilis is often accompanied by other systemic symptoms.
  • Drug Reactions: Certain medications can cause a range of skin reactions, including a full body blanching red rash, as part of a drug hypersensitivity reaction.