Differential Diagnosis for the Patient's Condition

The patient presents with a new development of erythematous plaques over the upper limbs, including hands and feet, after being treated with ceftriaxone and weekly methotrexate for pustular psoriasis. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Drug Rash (or Drug Eruption) to Methotrexate or Ceftriaxone: This is a common adverse reaction to medications, especially antibiotics and certain immunosuppressants like methotrexate. The timing of the rash, appearing a few days after starting the medications, supports this diagnosis.

• Other Likely Diagnoses

  • Psoriatic Exacerbation: The patient's underlying condition, pustular psoriasis, could be experiencing a flare, potentially triggered by the introduction of new medications or other factors.
  • Allergic Contact Dermatitis: Though less likely given the context, if the patient has been exposed to new substances (e.g., soaps, lotions) around the time of medication initiation, this could be a consideration.
  • Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): While more severe and less common, these conditions can be triggered by medications like ceftriaxone and methotrexate, especially if there's a history of sensitivity.

• Do Not Miss Diagnoses

  • DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms): A potentially life-threatening condition that can be triggered by medications. It presents with a rash, fever, lymphadenopathy, and internal organ involvement. Early recognition is crucial.
  • Sepsis: Given the patient's recent antibiotic use and the development of a new rash, sepsis, although unlikely, should be considered, especially if there are signs of systemic infection.

• Rare Diagnoses

  • Pustular Psoriasis Generalized: While the patient already has pustular psoriasis, a generalized form could present with widespread erythematous plaques and pustules, potentially exacerbated by the medications.
  • Acute Generalized Exanthematous Pustulosis (AGEP): A rare condition characterized by numerous small, sterile pustules on a background of erythema, often triggered by medications. It's distinct from pustular psoriasis and requires specific management.

Management

The management of the patient's condition would depend on the confirmed diagnosis. For a drug rash, discontinuation of the offending drug and symptomatic treatment may suffice. For more severe conditions like DRESS, SJS/TEN, or sepsis, immediate medical intervention, including hospitalization, may be necessary. Consultation with a dermatologist or an allergist could provide further guidance.