Differential Diagnosis for the 36-year-old Pregnant Female with a Rash

• Single most likely diagnosis:
  • Pityriasis rosea: This condition is characterized by a rash that initially appears as a single patch (herald patch) and later spreads to other areas, often in a Christmas tree distribution. The rash is typically uniformly red, with mild itching and sometimes burning sensations. The patient's history of a similar rash in the past that resolved on its own and the current description of the rash are consistent with pityriasis rosea.
• Other Likely diagnoses:
  • Contact dermatitis: Given the rash's location around the neck and its spread, contact dermatitis due to an allergen or irritant is possible. However, the uniform redness and lack of marked itchiness make this less likely.
  • Tinea corporis (ringworm): This fungal infection can cause ring-shaped rashes, but it typically presents with more pronounced itching and a clearer border between the affected and unaffected skin.
  • Viral exanthem: The patient's mild sore throat and the rash could suggest a viral etiology, but the lack of fever, chills, or arthralgias makes this less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Erythema multiforme: Although less common, this condition can present with target lesions and can be associated with herpes simplex virus infections or other viral infections. It requires prompt recognition and treatment due to potential complications.
  • Pemphigus: An autoimmune disease that can cause blistering skin lesions, pemphigus is rare but life-threatening if not treated. The patient's symptoms do not strongly suggest pemphigus, but it should be considered in the differential due to its severity.
  • Syphilis: Secondary syphilis can cause a rash that might fit the patient's description, and it is crucial to diagnose and treat syphilis in pregnancy due to the risk of congenital syphilis.
• Rare diagnoses:
  • Psoriasis: Although psoriasis can cause red, scaly patches, the patient's presentation and lack of a reported family history or previous episodes make this less likely.
  • Dermatomyositis: This autoimmune condition can cause skin rash and muscle weakness, but the patient's symptoms do not align closely with dermatomyositis, and it is relatively rare.