Differential Diagnosis for Raised Red Itchy Bumps
Single Most Likely Diagnosis
- Contact Dermatitis (Poison Ivy): The patient's history of poison ivy that started to get better, followed by the recurrence of raised red itchy bumps on the same arm and subsequent spread to other areas, strongly suggests a continued or new exposure to urushiol, the allergenic oil found in poison ivy. The pattern of spread and the description of the lesions are consistent with this diagnosis.
Other Likely Diagnoses
- Allergic Contact Dermatitis (other causes): Other substances could cause allergic contact dermatitis, such as nickel, latex, or certain chemicals in soaps or lotions, which might explain the spread of the rash if the patient came into contact with these allergens.
- Atopic Dermatitis (Eczema): Although less likely given the initial diagnosis of poison ivy, atopic dermatitis could be considered, especially if the patient has a history of eczema. The rash in atopic dermatitis can be itchy and appear as raised, red patches.
- Insect Bites: Multiple insect bites could cause a similar pattern of itchy, raised bumps, especially if the patient has been outdoors.
Do Not Miss Diagnoses
- Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Although rare and less likely given the description, these conditions are life-threatening and can initially present with skin lesions that resemble a rash. They are usually associated with medication use or infection.
- Cellulitis: A bacterial infection of the skin that can cause redness, swelling, and warmth. While the description doesn't fully align with cellulitis, it's crucial to consider given the potential for serious complications if not treated promptly.
Rare Diagnoses
- Pityriasis Rosea: A skin condition characterized by a rash that can appear as raised, red patches, often starting with a single "herald patch." It's less likely given the initial diagnosis of poison ivy but could be considered if the rash doesn't respond to typical treatments for contact dermatitis.
- Dermatologic Manifestations of Systemic Diseases: Certain systemic diseases, like lupus or rheumatoid arthritis, can have skin manifestations. These would be less likely without other systemic symptoms but should be considered in a comprehensive differential diagnosis.