Differential Diagnosis for Ovoid Rash with Vesicles under Arm

• Single most likely diagnosis
  • Herpes Zoster (Shingles): This is the most likely diagnosis due to the description of an ovoid rash with vesicles, which is characteristic of shingles, especially when it appears in a dermatomal distribution such as under the arm.
• Other Likely diagnoses
  • Contact Dermatitis: This could be a possibility if the rash is due to an allergic reaction or irritant under the arm, though vesicles are less common.
  • Eczema (Atopic Dermatitis): Can sometimes present with vesicles, especially in acute phases, and can occur in the axillary region.
  • Varicella (Chickenpox): Although typically more widespread, the initial presentation could be localized and might resemble an ovoid rash with vesicles.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing Fasciitis: Although rare and not typically presenting with an ovoid rash and vesicles initially, any rapidly progressing skin infection, especially in the axillary region, should prompt consideration of this diagnosis due to its high mortality rate.
  • Erythema Multiforme: A skin condition that can be triggered by infections or medications, characterized by target lesions, but can sometimes present with vesicles and should be considered due to its potential severity.
• Rare diagnoses
  • Pemphigus: An autoimmune disease that can cause blistering of the skin, which might present as vesicles, though it's less common under the arm and usually involves the mouth and other areas of the body.
  • Bullous Pemphigoid: Another autoimmune blistering disease, which could present with large vesicles or bullae, including in the axillary region, though it's less common.