Polymorphic Light Eruption (PMLE): This is the most common photodermatosis, characterized by an itchy, bumpy rash or blisters that appear after sun exposure, often on the knees, elbows, and other sun-exposed areas. The condition typically resolves on its own within a few days of avoiding further sun exposure.

Solar Urticaria: A rare condition where hives or wheals appear on sun-exposed skin, typically within minutes of exposure. The reaction is usually itchy and can be severe.
Berloque Dermatitis: A skin condition caused by the interaction between certain fragrances or perfumes and sunlight, leading to hyperpigmentation and dermatitis, which can resemble a rash.
Phototoxicity: A non-immunologic reaction to certain medications (like doxycycline, sulfonamides) or substances (such as St. John's Wort) that occurs when the skin is exposed to sunlight, resulting in a sunburn-like reaction.

Lupus Erythematosus: Systemic lupus erythematosus (SLE) can cause a photosensitive rash, often on the face (butterfly rash), but it can also appear on other sun-exposed areas like the knees. Missing this diagnosis could lead to delayed treatment of a potentially serious autoimmune disease.
Porphyria Cutanea Tarda: A disorder that results from a deficiency of the enzyme uroporphyrinogen decarboxylase, leading to blistering of the skin upon sun exposure. While rare, it's crucial to diagnose due to its implications for liver disease and potential neurological manifestations in other forms of porphyria.

Chronic Actinic Dermatitis: A rare condition characterized by a persistent, eczematous dermatitis in sun-exposed areas, more common in older males. It can be disabling and requires careful management.
Hydroa Vacciniforme: A rare photodermatosis of childhood, characterized by the appearance of vesicles and crusting lesions on sun-exposed areas, which can leave scars.
Actinic Prurigo: A rare, inherited condition that causes an itchy, bumpy rash on sun-exposed areas, more common in females and those of Native American descent.