Seborrheic Dermatitis: This is a common skin condition in elderly individuals, characterized by a maculopapular rash that can appear on the upper back. It is often associated with itching and can be exacerbated by stress, cold weather, and certain medications.

Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen or irritant, leading to a maculopapular rash. In an elderly woman, this could be due to sensitivity to laundry detergents, soaps, or other personal care products.
Eczema (Atopic Dermatitis): Although more common in children, eczema can persist into adulthood and may appear as a maculopapular rash on the upper back. It is characterized by intense itching and can be exacerbated by dry skin, stress, and allergens.
Drug Eruption: Many medications can cause skin rashes as a side effect. Given the high likelihood of polypharmacy in elderly patients, a drug eruption is a plausible diagnosis for a new-onset maculopapular rash.

Lyme Disease: Caused by the bacterium Borrelia burgdorferi, transmitted through tick bites. Early stages of Lyme disease can present with a characteristic "bull's-eye" rash (erythema migrans), but atypical presentations, including a maculopapular rash, can occur. Missing this diagnosis can lead to serious complications if left untreated.
Meningococcemia: This is a severe infection caused by Neisseria meningitidis. It can present with a petechial or purpuric rash, but early stages might appear as a blanching maculopapular rash. It is critical to identify and treat promptly due to its high mortality rate.
Rocky Mountain Spotted Fever (RMSF): Another tick-borne illness, RMSF can present with a maculopapular rash, fever, and headache. It is essential to consider this diagnosis in individuals who have been exposed to ticks, as delayed treatment can lead to severe complications.

Pityriasis Rosea: A condition characterized by a herald patch followed by a widespread, symmetrical rash. While it typically affects younger individuals, it can occur in the elderly and might present with a maculopapular appearance.
Secondary Syphilis: This can cause a wide range of skin manifestations, including a maculopapular rash. Although less common, it is a diagnosis to consider, especially in patients with risk factors for sexually transmitted infections.
Viral Exanthems: Certain viral infections, such as measles, rubella, or roseola, can cause maculopapular rashes. While these are more common in children, they can occur in adults, especially those who are immunocompromised or have not been vaccinated.