Erythema Migrans (Lyme Disease): This is the most common manifestation of early localized Lyme disease, typically presenting as a gradually expanding red patch, often with a "bull's-eye" appearance, although this is not always present. The patient's history of a suspected tick bite and the description of the lesion are consistent with this diagnosis. The fact that the lesion is itchy and has been present for 3 days also aligns with erythema migrans, which can be accompanied by itching and usually appears within a week of the tick bite.

Cellulitis: This is an infection of the skin and subcutaneous tissues, usually caused by bacteria. While it typically presents with redness, warmth, swelling, and sometimes pus, the absence of these latter features does not rule it out entirely, especially if the patient has a compromised immune system due to lupus.
Contact Dermatitis: Given the itchy nature of the lesion, contact dermatitis (an allergic reaction or irritation of the skin) could be considered, especially if the patient has come into contact with a new substance or plant.
Insect Bite Reaction: While the patient suspects a tick bite, other insect bites can cause significant reactions, including large, itchy lesions.

Southern Tick-Associated Rash Illness (STARI): Also known as Masters disease, this condition presents similarly to erythema migrans but is caused by a different tick-borne illness. It's crucial to consider this in the differential, especially if the patient has been in areas where the Lone Star tick is common.
Rocky Mountain Spotted Fever (RMSF): This is a serious tick-borne disease that can present with a rash, fever, and headache. Although the rash in RMSF typically appears later in the course of the disease and is often characterized by small, flat, pink, non-itchy spots, it's essential to consider RMSF due to its potential severity and the need for prompt treatment.
Babesiosis: Another tick-borne illness, babesiosis can present with non-specific symptoms such as fever and fatigue but can be severe, especially in immunocompromised individuals.

Ehrlichiosis/Anaplasmosis: These are tick-borne bacterial infections that can cause fever, headache, and sometimes a rash. They are less common than Lyme disease but should be considered, especially in patients with significant exposure to ticks.
Tularemia: Caused by the bacterium Francisella tularensis, tularemia can be transmitted by the bite of an infected tick. It typically presents with a skin ulcer and swollen lymph nodes, but a rash can also occur.
Southern Tick-Associated Rash Illness (STARI) with Systemic Symptoms: While STARI is generally considered to have a benign course, there have been reports of systemic symptoms, making it a rare but potentially significant diagnosis to consider.