**Core Concept**
The patient's symptoms suggest a chronic skin condition that worsens with exposure to environmental factors, particularly humidity and temperature changes. This condition is likely an allergic contact dermatitis or a photodermatosis.

**Why the Correct Answer is Right**
The patient's symptoms of itchy, excoriated papules on exposed areas, particularly the forehead and arms, suggest a photodermatosis. The fact that the condition worsens in the rainy season and improves in winter suggests a condition that is exacerbated by humidity and temperature changes. The most likely diagnosis is solar urticaria or polymorphic light eruption, but given the long duration and the specific distribution of the lesions, a more likely diagnosis is solar dermatitis or a variant of polymorphic light eruption.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not take into account the patient's symptoms or the environmental factors that exacerbate the condition.
* **Option B:** This option is incorrect because it is too broad and does not specify the type of dermatitis.
* **Option C:** This option is incorrect because it does not consider the specific distribution of the lesions or the environmental factors that exacerbate the condition.

**Clinical Pearl / High-Yield Fact**
Polymorphic light eruption (PMLE) is a common photodermatosis that can present with a variety of symptoms, including itchy papules, plaques, and vesicles. It is more common in individuals with fair skin and can be exacerbated by humidity and temperature changes.

**Correct Answer: D. Polymorphic light eruption.**