Contact Dermatitis: This is the most likely diagnosis given the patient's history of working in the yard and the onset of the rash on Monday evening. The rash's description as extremely pruritic with some crusting and its location on the bilateral lower extremities and forearms also support this diagnosis. Contact dermatitis is a common condition that occurs when the skin comes into contact with an allergen or irritant, leading to an immune response and subsequent inflammation.

Insect Bites or Stings: The patient's outdoor activities could have exposed her to insects, and the rash's pruritic nature could be consistent with an allergic reaction to bites or stings.
Allergic Reaction to Plants: Similar to contact dermatitis, an allergic reaction to certain plants (e.g., poison ivy, oak, or sumac) could cause a rash after outdoor exposure.
Eczema (Atopic Dermatitis): Although less likely given the acute onset, eczema could be considered, especially if the patient has a history of atopic diseases.

Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although the patient denies any lip swelling, difficulty swallowing, or rash on the face or genitals, these conditions can rapidly progress and are life-threatening. They are typically associated with medication use or infections.
Cellulitis: A bacterial infection of the skin that could present with similar symptoms, including redness, swelling, and warmth of the skin. It requires prompt antibiotic treatment to prevent serious complications.

Phototoxic or Photoallergic Reaction: A reaction to sunlight, either due to a medication or a plant, could cause a rash. However, this would typically involve areas exposed to the sun, which might not entirely match the patient's presentation.
Pityriasis Rosea: A viral rash that can appear after outdoor activities, though its characteristic "herald patch" followed by a secondary eruption in a "Christmas tree" distribution is not mentioned in the scenario.