A 58-year-old woman showing symptoms of tachycardia, S3 heart sounds, and basilar crackles with a hypotensive presentation is most likely experiencing an exacerbation of her pre-existing congestive heart failure.
The 58-year-old woman with congestive heart failure (CHF) presents with tachycardia, an S3 heart sound, and basilar crackles bilaterally with a low blood pressure of 90/68 mm Hg. These clinical findings suggest an exacerbation of her underlying heart condition rather than other diseases such as aortic stenosis, myocardial infarction, or a pulmonary embolism.
The S3 heart sound and bilateral basilar crackles are indicative of fluid congestion typically seen in congestive heart failure. The fact that she already has a history of CHF along with hypoxemia and these symptoms supports this diagnosis.
So, the most likely diagnosis for this patient with her current presentation is congestive heart failure (D).