Differential Diagnosis for a 56-year-old Female with a Blistering Painful Rash after Hiking

Single Most Likely Diagnosis

• Contact Dermatitis: This is the most likely diagnosis given the history of hiking, which increases the risk of exposure to allergens or irritants such as plants (e.g., poison ivy, oak, or sumac), insects, or other environmental factors. The presentation of a blistering painful rash is consistent with an allergic contact dermatitis reaction.

Other Likely Diagnoses

• Insect Bites or Stings: Various insects encountered during hiking can cause blistering rashes, including mosquitoes, spiders, and certain types of flies. The reaction can range from mild to severe.
• Phototoxicity or Photoallergic Reaction: Prolonged sun exposure during hiking can lead to phototoxicity or a photoallergic reaction, especially if the patient has taken certain medications (e.g., doxycycline, sulfonamides) that increase sensitivity to sunlight.
• Fungal Infections: Fungal infections like ringworm (tinea corporis) can present with blistering lesions, although they are less common in this context.

Do Not Miss Diagnoses

• Lyme Disease: Although it typically presents with a characteristic "bull's-eye" rash (erythema migrans), Lyme disease can sometimes cause a blistering rash. Given the potential for severe consequences if untreated, it's crucial to consider this diagnosis, especially if the patient has been in areas where Lyme disease is endemic.
• Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can present with a rash, which may be blistering in some cases. RMSF is a medical emergency due to the risk of severe complications and death if not promptly treated.
• Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually triggered by medications or infections. They can present with blistering skin lesions and are medical emergencies due to their high mortality rate.

Rare Diagnoses

• Leptospirosis: A zoonotic infection that can be acquired through contact with water contaminated with the urine of infected animals. It can cause a rash among other symptoms, but this is less common.
• Erythema Multiforme: An immune-mediated disorder that can be triggered by infections or medications, characterized by target-like lesions, which may blister.

Step-by-Step Workup or Testing

1. Detailed History: Gather information about the patient's hike, including the location, duration, and any potential exposures to plants, insects, or water.
2. Physical Examination: Carefully examine the rash, noting its distribution, morphology, and any associated symptoms like fever or lymphadenopathy.
3. Laboratory Tests:
   • Complete Blood Count (CBC): To assess for signs of infection or inflammation.
   • Blood Cultures: If there's suspicion of a systemic infection like Lyme disease or RMSF.
   • Serologic Tests: For Lyme disease or other tick-borne illnesses if suspected.
   • Skin Biopsy: In cases where the diagnosis is unclear or to confirm conditions like Stevens-Johnson Syndrome.
4. Imaging: May not be necessary for the initial evaluation of a rash but could be useful in certain contexts, such as assessing for signs of systemic infection.
5. Allergy Testing: If contact dermatitis is suspected and the offending agent is not clear, patch testing may be considered.

This approach allows for a systematic evaluation of the patient's condition, ensuring that both common and potentially life-threatening diagnoses are considered and appropriately investigated.