Differential Diagnosis for Pruritic Rash on Posterior Thighs
- Single most likely diagnosis
- Contact dermatitis: This is the most likely diagnosis given the patient's symptoms of a pruritic rash on the posterior thighs. Although she denies any change in detergents, soaps, or creams, and does not recall direct exposure to poison ivy, contact dermatitis can occur from various sources, including plants, metals, or other substances she might have come into contact with at Hershey Park, even if indirectly (e.g., through touching surfaces that had been in contact with an allergen).
- Other Likely diagnoses
- Scabies: Given the pruritic nature of the rash and the fact that she sat on a toilet at Hershey Park, scabies is a possibility. Scabies is highly contagious and can be spread through direct skin-to-skin contact or, less commonly, through fomites.
- Insect bites: Although she was wearing long pants, it's possible she was bitten by insects at some point during her outing, leading to a pruritic rash.
- Allergic reaction to an unknown substance: The patient's concern about picking something up from the toilet suggests an awareness of potential environmental exposures. An allergic reaction to an unknown substance she came into contact with cannot be ruled out without further investigation.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Bed bugs: While less likely, bed bugs can cause pruritic rashes and are associated with public places, including amusement parks and their facilities. Given the severity of infestation and the difficulty in eradicating them, missing this diagnosis could lead to significant distress and health issues.
- Lyme disease or other tick-borne illnesses: Although she denies direct exposure to poison ivy and was wearing long pants, the possibility of a tick bite, especially in an outdoor setting like Hershey Park, cannot be entirely ruled out. Early recognition of tick-borne illnesses is crucial for effective treatment.
- Rare diagnoses
- Phototoxic or photoallergic reaction: If she was exposed to certain substances (like some medications or plant allergens) and then had sun exposure, a phototoxic or photoallergic reaction could occur, though this seems less likely given her clothing and lack of reported sun exposure.
- Zoonotic infections: While rare, certain zoonotic infections (diseases transmitted from animals to humans) could potentially cause a rash, especially if she had contact with animals at the park, though this is highly speculative without further information.