Differential Diagnosis for Possible Left Foot Infection

Single Most Likely Diagnosis

• Cellulitis: This is the most likely diagnosis given the presence of erythema, warmth, and purulence on the underside of the foot. Cellulitis is a common bacterial skin infection that presents with these symptoms, and the absence of fluctuance suggests that it has not progressed to an abscess.

Other Likely Diagnoses

• Abscess: Although there is no fluctuance, an abscess cannot be completely ruled out without further imaging or exploration. The presence of purulence suggests a possible collection of pus that could be indicative of an abscess.
• Ulcer Infection: Given the location on the underside of the foot, there could be an underlying ulcer (possibly diabetic or traumatic) that has become infected, presenting with similar symptoms.
• Erysipelas: A type of skin infection that involves the upper dermis, typically caused by beta-hemolytic group A Streptococcus. It presents with well-demarcated erythema and can be considered given the patient's symptoms.

Do Not Miss Diagnoses

• Necrotizing Fasciitis: Although less likely, this is a life-threatening condition that requires immediate attention. The presence of erythema, warmth, and purulence, along with the potential for rapid progression, makes it crucial not to miss this diagnosis.
• Gas Gangrene: Another severe condition that could present with similar symptoms, especially if there is a wound or trauma to the foot. It is characterized by the production of gas in the tissues and requires prompt treatment.
• Osteomyelitis: An infection of the bone that could be underlying the soft tissue infection. It might not be immediately apparent but is crucial to consider, especially in patients with risk factors such as diabetes or a history of foot trauma.

Rare Diagnoses

• Erythema Elevatum Diutinum (EED): A rare skin condition characterized by red, purple, or brown lesions that can appear anywhere on the body. It's less likely but could be considered if other diagnoses are ruled out.
• Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by fever, neutrophilia, and tender, erythematous skin lesions. It's rare and might not be the first consideration but should be kept in mind if the patient's condition does not respond to typical treatments for infection.