A 75-year-old woman with non-small cell lung carcinoma presents with confusion and cognitive impairment. What is the most likely diagnosis given her symptoms and low sodium levels?

In this scenario, the patient presents with confusion and cognitive impairment, alongside low sodium levels (hyponatremia). Given these symptoms, the most plausible diagnosis is delirium.

Delirium is characterized by an acute onset of confusion and changes in cognitive function. It often occurs in the setting of an underlying medical condition, such as infections, metabolic disturbances, or medication side effects. In this case, the low sodium levels can contribute to altered mental status, leading to rapid cognitive impairment and confusion, which aligns with the features of delirium.

The acute presentation and the presence of a metabolic derangement, namely hyponatremia, strongly support the diagnosis of delirium over chronic conditions such as dementia, where cognitive decline progresses gradually and typically without acute fluctuations.