Differential Diagnosis for Rash on Hands and Feet

The patient presents with a rash on the hands and feet, characterized by targetoid macules with dark red centers, which causes discomfort but does not itch. Given the patient's history and physical examination findings, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis

  • Erythema Multiforme (EM): This condition is characterized by the presence of targetoid lesions, which are sharply demarcated and have a dark red center, matching the patient's presentation. The recent diagnosis of herpes simplex and the use of doxycycline two weeks prior could be triggering factors for EM. The condition is known for its association with infections, including herpes simplex, and certain medications.

• Other Likely Diagnoses

  • Psoriasis: Given the family history of psoriasis and the presence of lesions on the palms and soles, psoriasis could be considered, especially if the lesions evolve to show more characteristic psoriatic features like scaling. However, the targetoid appearance and the acute onset are less typical for psoriasis.
  • Lupus Erythematosus: With a family history of lupus erythematosus, this condition could be considered, especially if the patient exhibits other systemic symptoms. However, the lack of other typical lupus symptoms (e.g., joint pain, fever, malar rash) and the specific lesion characteristics make it less likely.
  • Stevens-Johnson Syndrome (SJS): Although SJS is a more severe condition that can present with targetoid lesions, the patient's symptoms seem milder, and there's no mention of mucosal involvement or a more severe systemic reaction, which are hallmarks of SJS.

• Do Not Miss Diagnoses

  • Toxic Epidermal Necrolysis (TEN): This is a severe skin condition usually triggered by medications. Although the patient's symptoms seem less severe, any sign of skin detachment or widespread skin involvement would necessitate immediate consideration of TEN due to its high mortality rate.
  • Infectious Etiologies (e.g., Strep-related rash): Given the recent outbreak of strep pharyngitis in the patient's daycare center, an infectious cause should be considered, especially if the rash is accompanied by other symptoms like fever or sore throat.

• Rare Diagnoses

  • Rowell Syndrome: A rare condition characterized by lupus erythematosus and erythema multiforme-like lesions. Given the family history of lupus, this could be considered, but it's much less common.
  • Secondary Syphilis: Can present with a variety of skin rashes, including on the palms and soles. However, the targetoid appearance and the lack of other syphilitic symptoms make this less likely without further diagnostic evidence.