Hypertensive Emergency (e.g., Hypertensive Encephalopathy or Intracranial Hemorrhage): The patient's extremely high blood pressure (305/120 mmHg) combined with symptoms of chest pain and declining mental status suggests a hypertensive emergency. The high blood pressure can lead to end-organ damage, including the brain, heart, and kidneys. The chest pain could be indicative of cardiac involvement, such as myocardial infarction or aortic dissection, while declining mental status could suggest cerebral edema or hemorrhage.

Acute Coronary Syndrome (Myocardial Infarction): The chest pain and high blood pressure could be indicative of an acute coronary syndrome, where the high blood pressure increases the demand on the heart, potentially leading to a myocardial infarction.
Aortic Dissection: The combination of severe hypertension and chest pain raises the suspicion for an aortic dissection, a condition where there is a tear in the inner layer of the aorta.
Pulmonary Embolism: Although less directly related to the high blood pressure, the chest pain and declining mental status could also be symptoms of a large pulmonary embolism, especially if the patient has risk factors for thromboembolic disease.

Subarachnoid Hemorrhage: A sudden, severe headache (though not explicitly mentioned, could be part of the chest pain complaint) and declining mental status in the context of high blood pressure should prompt consideration of a subarachnoid hemorrhage, which is immediately life-threatening.
Cardiac Tamponade: Although less common, cardiac tamponade could present with chest pain and hypotension (not explicitly present but could develop), and it's crucial to consider in the differential due to its high mortality if not promptly treated.
Pneumothorax: A tension pneumothorax could cause chest pain and declining mental status due to decreased cardiac output and is a medical emergency.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension, along with other symptoms like chest pain and palpitations. It's less likely but should be considered in the differential of hypertensive crises, especially if other causes are ruled out.
Thyroid Storm: Although rare, thyroid storm can cause severe hypertension, tachycardia, and altered mental status, making it a differential diagnosis in this scenario, especially if there's a history of thyroid disease.
Eclampsia: In a pregnant patient, eclampsia (seizures in a woman with preeclampsia) could present with high blood pressure and altered mental status, making it a critical diagnosis to consider in pregnant women.