Urinary Tract Infection (UTI): In the elderly, UTIs can present atypically, with symptoms such as acute lethargy, confusion, or altered mental status, rather than the typical symptoms of dysuria or frequency seen in younger patients.

Dehydration: Dehydration is common in the elderly due to various factors like decreased fluid intake, increased fluid loss, or comorbid conditions. It can lead to lethargy, confusion, and other neurological symptoms.
Medication Side Effects: Elderly patients often take multiple medications, which can have side effects such as sedation, confusion, or lethargy, especially when doses are adjusted or new medications are added.
Electrolyte Imbalance: Conditions like hyponatremia, hypernatremia, hypokalemia, or hyperkalemia can cause lethargy among other symptoms in the elderly.
Pneumonia: Community-acquired or hospital-acquired pneumonia can present with non-specific symptoms in the elderly, including lethargy, especially if there is an underlying condition affecting the brain or if the patient has a decreased ability to mount a typical inflammatory response.

Stroke or Transient Ischemic Attack (TIA): Although more commonly associated with focal neurological deficits, stroke or TIA can sometimes present with non-specific symptoms like lethargy, especially in the context of decreased level of consciousness.
Subdural Hematoma: Falls are common in the elderly, and even minor trauma can lead to subdural hematoma, which may present with altered mental status or lethargy.
Sepsis: Sepsis, particularly in its early stages, can manifest with non-specific symptoms such as lethargy. Given its high mortality rate, early recognition is crucial.
Hypoglycemia: Especially in diabetic patients, hypoglycemia can cause lethargy, confusion, and loss of consciousness. It is easily treatable but can be fatal if not recognized promptly.

Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by altered mental status, including lethargy, which can be reversible with treatment.
Vasculitis: Certain types of vasculitis, such as giant cell arteritis or primary central nervous system vasculitis, can present with non-specific symptoms including lethargy, although they are much less common.
Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause rapid deterioration in mental and physical abilities, including lethargy, though it typically presents with more distinctive symptoms like myoclonus and ataxia.