Differential Diagnosis for Sudden Inflammation and Redness of the Foot

Single Most Likely Diagnosis

• Cellulitis: This is the most likely diagnosis given the sudden onset of inflammation and redness, especially with an apparent bite on the foot. Cellulitis is a common bacterial skin infection that can occur after a bite or other skin break, leading to redness, swelling, and warmth of the affected area.

Other Likely Diagnoses

• Spider Bite (e.g., Brown Recluse): While less common, a spider bite could cause significant inflammation and necrosis, especially if it's from a venomous spider like the Brown Recluse. The presence of a bite mark supports this possibility.
• Insect Bite Reaction: An allergic reaction to an insect bite could also present with inflammation and redness, although it might not typically be as severe as cellulitis or a spider bite.
• Erythema Infectiosum (Fifth Disease): Although less likely in adults, Fifth Disease can cause a distinctive "slapped cheek" appearance and can sometimes affect the feet, but it usually starts with a fever and upper respiratory symptoms.

Do Not Miss Diagnoses

• Necrotizing Fasciitis: This is a life-threatening condition that requires immediate attention. It can start with symptoms similar to cellulitis but progresses rapidly, causing severe pain, swelling, and necrosis of the skin and underlying tissues. The presence of a bite could be a portal of entry for the bacteria.
• Gas Gangrene: Another potentially deadly condition, gas gangrene can occur after a traumatic wound (including bites) becomes infected with Clostridium perfringens. Symptoms include severe pain, swelling, and gas in the tissues.
• Deep Vein Thrombosis (DVT): While less directly related to a bite, inflammation and redness in one leg could also suggest a DVT, especially if there's significant swelling and pain. DVT is a medical emergency due to the risk of pulmonary embolism.

Rare Diagnoses

• Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet. It could be considered if other diagnoses are ruled out, but it doesn't typically start with a bite.
• Complex Regional Pain Syndrome (CRPS): This is a chronic pain condition that can follow an injury (including a bite) and is characterized by severe, burning pain and changes in skin temperature and color. It's rare and would be a diagnosis of exclusion.