After a camping trip, a patient presents with a rash on the limbs and trunk and a small painful blister near the inner thigh. Based on this presentation, which condition should you suspect?

• A. Lyme disease
• B. Poison ivy dermatitis
• C. Ringworm infection
• D. Psoriasis

Answer: (A)

Exposure to ticks after camping points strongly toward a tick-borne illness. The early localized stage of Lyme disease often presents with an expanding rash at the bite site, called erythema migrans, which can appear on the limbs or trunk and may be accompanied by a small tender blister at the bite. This pattern—recent outdoor exposure with a rash that originated near a bite site and is spreading—is the best fit for Lyme disease.

Poison ivy dermatitis, by contrast, is an allergic contact reaction that tends to be intensely itchy and appears as vesicles in areas of contact with the plant, often in a linear or streaked pattern. Ringworm (tinea) typically shows circular, scaly patches with central clearing, not a spreading rash from a tick bite. Psoriasis presents as chronic, well-demarcated plaques with silvery scales, not the acute, bite-site–originating rash seen after a camping trip.

So the presentation aligns with Lyme disease due to tick exposure and the characteristic expanding rash from a bite site. If suspected, seek medical evaluation for appropriate management.