Differential Diagnosis for Pustular Rash on the Left Knee

• Single most likely diagnosis
  • Folliculitis: This is a common condition characterized by inflammation of the hair follicles, often caused by bacterial infection, which can present as a pustular rash. The knee area, with its high density of hair follicles, is a common site for folliculitis.
• Other Likely diagnoses
  • Acne: Although more commonly associated with the face, acne can occur on other parts of the body, including the knees, especially in individuals prone to acne. It can present with pustules.
  • Impetigo: A highly contagious skin infection that can cause pustular lesions, often starting as red sores on the face but can spread to other areas, including the knees, through touch or clothing.
  • Contact Dermatitis: An allergic or irritant reaction to substances coming into contact with the skin, which can sometimes present with pustules, especially if there's a secondary infection.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Cellulitis: A bacterial skin infection that can cause redness, swelling, and sometimes pustules. It requires prompt antibiotic treatment to prevent serious complications.
  • Necrotizing Fasciitis: A severe infection of the tissue beneath the skin, including fat and the tissue covering the muscles (fascia). It's a medical emergency and can initially present similarly to cellulitis but with more severe pain and systemic symptoms.
• Rare diagnoses
  • Pustular Psoriasis: A rare type of psoriasis that can cause pus-filled bumps on the skin, often accompanied by redness and scaling. It can be localized or generalized.
  • Subcorneal Pustular Dermatosis: A rare skin condition characterized by the formation of pustules just beneath the outer layer of the skin, often without an identifiable cause.
  • Acute Generalized Exanthematous Pustulosis (AGEP): A rare skin condition usually triggered by medications, characterized by the rapid appearance of many small pustules on the skin, often accompanied by fever and systemic symptoms.