Scabies: The description of a blistery rash with a central red mark and an excoriated, scabbed appearance, particularly itchy at bedtime, is highly suggestive of scabies. The fact that the rash may have initially started on her scalp and the intense itching, especially at night, further supports this diagnosis. Scabies is common in institutional settings like assisted living facilities.

Contact Dermatitis: Given the patient's history of doing her own laundry, it's possible she might have come into contact with a new detergent or fabric softener that could cause an allergic reaction, leading to a rash. However, the absence of new skin products and the specific description of the rash make this less likely.
Eczema (Atopic Dermatitis): This condition can cause itchy, blistering rashes, especially in older adults. The fact that the rash is particularly itchy at bedtime could be consistent with eczema, but the specific pattern and progression of the rash are less typical.
Insect Bites: The description of the rash could also be consistent with insect bites, especially if the patient has been exposed to biting insects. However, the pattern and the fact that the rash started on the scalp make this less likely.

Bullous Pemphigoid: This is an autoimmune disease that can cause blistering skin lesions. It's more common in older adults and can be mistaken for other skin conditions. Although less likely, missing this diagnosis could lead to significant morbidity if not treated appropriately.
Herpes Zoster (Shingles): While the patient does not report pain, which is a common symptom of shingles, the blistering rash could be consistent with this diagnosis, especially if the patient is immunocompromised or has not received the shingles vaccine. Missing this diagnosis could lead to serious complications, including postherpetic neuralgia.

Pemphigus Vulgaris: This is a rare autoimmune disease that causes blistering of the skin and mucous membranes. It's less likely given the patient's presentation but should be considered if other diagnoses are ruled out.
Dermatitis Herpetiformis: This condition is characterized by a blistering, itchy skin rash and is associated with celiac disease. It's rare and less likely given the patient's presentation, but it could be considered if other diagnoses are excluded and there's a suspicion of celiac disease.