Contact Dermatitis: This is the most likely diagnosis due to the linear and welt-like appearance of the rash, which is consistent with an allergic reaction to something that came into contact with the skin, such as a plant (e.g., poison ivy, oak, or sumac) or an irritant.

Insect Bites or Stings: Linear patterns can also be seen with bites or stings from insects like bed bugs or mosquitoes, especially if the patient has scratched the area, leading to linear excoriations.
Phytocontact Dermatitis: Similar to contact dermatitis but specifically caused by plants. The linear pattern is often seen due to the brushing against leaves or stems of certain plants.
Linear IgA Disease: An autoimmune condition that can present with linear blistering lesions, though it's less common than other causes of linear rashes.

Staphylococcal Scalded Skin Syndrome (SSSS): Although SSSS typically presents with more widespread skin exfoliation, in its early stages, it can manifest as linear, welt-like lesions, especially in children. It's crucial to consider this diagnosis due to its potential severity and need for prompt antibiotic treatment.
Toxic Epidermal Necrolysis (TEN): A severe skin condition usually triggered by medications. Early stages might resemble a linear rash, and it's critical to identify and treat promptly due to its high mortality rate.

Dermatitis Herpetiformis: An autoimmune blistering disorder associated with celiac disease, which can occasionally present with linear or grouped vesicles, though it's more commonly seen on extensor surfaces.
Linear Scleroderma: A rare condition characterized by linear bands of sclerotic skin, which can sometimes mimic a welt-like rash in its early stages, though it typically progresses to show more characteristic skin hardening and atrophy.