Differential Diagnosis for Red Tender Firm Back Lesion

• Single most likely diagnosis
  • Furuncle (Boil): A furuncle is a deep-seated infection of the hair follicle, typically caused by Staphylococcus aureus. It presents as a red, tender, and firm lesion, which matches the description provided. The location on the back, an area prone to sweating and friction, increases the likelihood of a furuncle.
• Other Likely diagnoses
  • Abscess: An abscess is a collection of pus that has built up within the tissue of the body. It can appear as a red, tender, and firm lesion. The back is a common location for abscesses due to its large surface area and potential for trauma or infection.
  • Infected Epidermoid Cyst: Epidermoid cysts are benign cysts that can become infected, leading to redness, tenderness, and firmness. The back is a common location for these cysts, and infection would explain the symptoms described.
  • Cellulitis: Cellulitis is a bacterial infection of the skin and subcutaneous tissues. It can present as a red, tender area that is firm to the touch. While it tends to be more diffuse than a single lesion, it could be considered in the differential diagnosis for a red tender firm back lesion.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing Fasciitis: Although rare, necrotizing fasciitis is a life-threatening infection that requires immediate medical attention. It can start as a red, tender, and firm area of skin but rapidly progresses. Early diagnosis is crucial for survival.
  • Spinal Epidural Abscess: This is a collection of pus that forms in the space between the skull and the spinal cord (epidural space). It can present with back pain and may have a localized area of redness and tenderness over the spine. It is a medical emergency due to the risk of spinal cord compression and paralysis.
• Rare diagnoses
  • Erythema Nodosum: A type of skin inflammation that is located in a certain portion of the fatty layer of skin. It results in tender red nodules or patches, most commonly on the shins. While less common on the back, it could be considered in the differential diagnosis.
  • Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous, well-demarcated papules and plaques. It is rare and would be an unusual cause of a single red tender firm lesion on the back.