Eczenatous Dermatitis (Eczema): Given the age and presentation of a pruritic rash with red papules on the chest, eczematous dermatitis is a common condition that can occur at any age, including the elderly. It is characterized by inflammation of the skin, leading to dryness, itchiness, and the appearance of red papules or plaques.

Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen or irritant, leading to a pruritic rash. In elderly individuals, the skin's barrier function may be compromised, making them more susceptible to contact dermatitis.
Seborrheic Dermatitis: Although more commonly associated with oily areas of the body like the scalp, seborrheic dermatitis can also occur on the chest, especially in older adults. It presents with a pruritic, erythematous rash.
Pityriasis Rosea: This condition is characterized by a herald patch followed by a secondary eruption of oval-shaped, scaling papules. While it can occur at any age, it's less common in the elderly but should be considered due to its distinctive presentation.

Bullous Pemphigoid: An autoimmune blistering disease that can present with pruritic urticarial plaques and bullae. It is more common in the elderly and can be life-threatening if not treated promptly.
Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A type of skin cancer that can mimic various dermatoses, including eczema. It's crucial to consider this diagnosis, especially if the rash is persistent and not responding to standard treatments.
Drug Eruption: Given the high likelihood of polypharmacy in an 89-year-old, drug eruptions should be considered. These can range from morbilliform rashes to more severe reactions like Stevens-Johnson syndrome or toxic epidermal necrolysis.

Pemphigus Vulgaris: Another autoimmune blistering disease that can present with skin lesions, though it more commonly involves mucous membranes. It's rare but potentially life-threatening.
Dermatitis Herpetiformis: Characterized by intensely pruritic, papulovesicular lesions, this condition is associated with celiac disease. It's less common and might be considered if other diagnoses are ruled out and there's a suspicion of gluten sensitivity.
Erythema Multiforme: An immune-mediated disorder that can be triggered by infections or medications, presenting with targetoid lesions. While it can occur at any age, it's relatively rare in the elderly without a clear precipitant.