Delirium: This is the most likely diagnosis given the patient's age and presentation with hallucinations and confusion. Delirium is a common condition in elderly patients, often triggered by medical illnesses, medications, or environmental changes.

Dementia with Psychotic Features: Patients with dementia can experience hallucinations and confusion, especially in the later stages of the disease.
Urinary Tract Infection (UTI) with Negative UA: Although the UA is negative, a UTI cannot be completely ruled out, especially in elderly patients who may not present with typical symptoms. A urine culture should be considered.
Medication-Induced Confusion: Many medications can cause confusion and hallucinations in elderly patients, including sedatives, anticholinergics, and certain antibiotics.

Central Nervous System (CNS) Infection: Conditions like meningitis or encephalitis can present with confusion and hallucinations and are medical emergencies.
Subdural Hematoma: Elderly patients are at risk for falls, and a subdural hematoma can cause confusion and hallucinations.
Hyperthyroidism or Hypothyroidism: Thyroid disorders can cause cognitive symptoms, including confusion and hallucinations.
Vitamin B12 Deficiency: This can cause neurological symptoms, including confusion and hallucinations.

Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause dementia, hallucinations, and confusion.
Neurosyphilis: A rare condition that can cause a range of neurological symptoms, including confusion and hallucinations.
Lewy Body Dementia: A progressive neurodegenerative disorder characterized by the presence of Lewy bodies in the brain, which can cause hallucinations and confusion.

The workup should include:

Complete medical history and physical examination
Laboratory tests: complete blood count (CBC), electrolytes, blood urea nitrogen (BUN), creatinine, glucose, liver function tests, thyroid function tests, and vitamin B12 level
Urine culture
Imaging studies: head CT or MRI if CNS infection or subdural hematoma is suspected
Consider EEG if seizures are suspected

Treatment will depend on the underlying cause but may include:

Managing delirium with environmental modifications and possibly haloperidol or other antipsychotics
Antibiotics if a UTI or CNS infection is diagnosed
Adjusting medications that may be contributing to confusion
Thyroid hormone replacement if hypothyroidism is diagnosed
Vitamin B12 supplementation if deficiency is found
Supportive care for rare diagnoses like Creutzfeldt-Jakob disease or neurosyphilis.