Differential Diagnosis for Sudden Redish Spot

• Single most likely diagnosis
  • Petechiae: Small, pinpoint red spots that occur due to minor bleeding from broken capillary blood vessels, often seen in conditions like thrombocytopenia or after strenuous exercise.
• Other Likely diagnoses
  • Erythema: A general term for redness of the skin, which can be caused by a variety of factors including infection, allergic reactions, or inflammation.
  • Urticaria (Hives): Itchy, raised welts that can appear anywhere on the body, often due to allergic reactions.
  • Insect Bites: Red spots or bumps that appear after being bitten by insects like mosquitoes or bed bugs.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Meningococcal Septicaemia: A severe infection caused by Neisseria meningitidis, characterized by a rash that can start as small red spots and rapidly progress, potentially leading to sepsis and death if not promptly treated.
  • Rocky Mountain Spotted Fever: A tick-borne disease that can cause a characteristic rash, starting as small red spots, and can be fatal if not treated early.
• Rare diagnoses
  • Erythema Multiforme: A skin condition characterized by target-shaped lesions, often triggered by infections or medications.
  • Henoch-Schönlein Purpura: A form of vasculitis that affects blood vessels and can cause a purpuric rash (red spots that do not blanch with pressure), typically seen in children.