Depression: Given the patient's history of significant medical conditions, including CVA with left-sided hemiparesis, multiple TIAs, seizure disorder, and congestive heart failure, along with his mostly bedbound state and increasing sleep, depression is a highly plausible cause of his worsening fatigue. The use of mirtazapine, an antidepressant, suggests that depression is already being considered, but the dosage might need adjustment or additional therapeutic strategies might be necessary.

Medication Side Effects: The patient is on multiple medications, including lorazepam and mirtazapine, which can cause sedation and contribute to fatigue. Reviewing and possibly adjusting his medication regimen could help alleviate fatigue.
Deconditioning: Being mostly bedbound and only getting out of bed for about an hour a day can lead to significant deconditioning, contributing to fatigue. Encouraging increased physical activity, even if minimal, could be beneficial.
Urinary Tract Infections (UTIs) and Potential Sepsis: Recurring UTIs can cause significant fatigue, especially in elderly patients. Ensuring that the current UTI is adequately treated and preventing future infections could help reduce fatigue.
Anemia: Although the patient has had stable hemoglobin levels since his last hospitalization, anemia is a common cause of fatigue in the elderly and should be ruled out with a recent blood count.

Heart Failure Exacerbation: Despite the patient's congestive heart failure being described as stable, worsening fatigue could be an early sign of decompensation. Monitoring for signs of heart failure exacerbation and adjusting medications as needed is crucial.
Infection or Sepsis: Beyond UTIs, other infections could be causing or contributing to the patient's fatigue. A thorough evaluation for any signs of infection is necessary.
Electrolyte Imbalance: Medications like furosemide can lead to electrolyte imbalances (e.g., hypokalemia, hyponatremia), which can cause fatigue among other symptoms. Regular electrolyte checks are important, especially in patients on diuretics.

Sleep Apnea: Although less likely, sleep apnea could contribute to the patient's increasing sleepiness and fatigue. Given his age and medical history, evaluating his sleep patterns might be beneficial.
Endocrine Disorders: Conditions like hypothyroidism or adrenal insufficiency can cause fatigue. While less common, these should be considered if other causes are ruled out, especially given the patient's complex medical history.
Neurodegenerative Diseases: The progression of his CVA or the development of other neurodegenerative diseases could explain worsening fatigue, though this would be more of a long-term consideration rather than an acute cause.