**Core Concept**
Urticaria is a common skin condition characterized by the appearance of wheals, which are well-circumscribed, edematous papules that vary in size. These lesions are typically itchy and can appear anywhere on the body, including the back, legs, and trunk. The condition is often associated with an allergic reaction or a release of histamine from mast cells.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with urticaria, which is commonly triggered by exposure to allergens such as insect bites, medications, or in this case, a lake environment. The release of histamine from mast cells leads to vasodilation, increased vascular permeability, and the characteristic wheals. The presence of erythematous, edematous papules and the absence of mucosal lesions or lip swelling further support this diagnosis. The variable size of the wheals is also consistent with urticaria.

**Why Each Wrong Option is Incorrect**
**Option B:** Folliculitis is an inflammation of the hair follicles, typically caused by bacterial or fungal infections. It is characterized by pustules or pus-filled bumps, which are not consistent with the patient's symptoms.

**Option C:** Erythema multiforme is a skin condition characterized by target-shaped lesions, which are not present in this case. It is often associated with infections, medications, or autoimmune disorders.

**Option D:** Erythema chronicum migrans is a skin lesion that appears as a slowly expanding, red patch, often associated with Lyme disease. It typically appears at the site of a tick bite and is not consistent with the patient's symptoms.

**Clinical Pearl / High-Yield Fact**
Urticaria can be caused by a wide range of triggers, including medications, foods, insects, and environmental factors. A thorough history and physical examination are essential in identifying the underlying cause of the condition.

**✓ Correct Answer: A. Urticaria**