Systemic Lupus Erythematosus (SLE) with Malar Rash: The combination of facial flush and SLE is highly suggestive of a malar rash, which is a characteristic "butterfly-shaped" rash across the cheeks and nose, often seen in patients with SLE. This rash can be triggered or worsened by sunlight, leading to facial flushing.

Rosacea: A chronic skin condition characterized by facial flushing, redness, and visible blood vessels. It can be exacerbated by various factors, including sun exposure, stress, and certain medications.
Polycythemia Vera: A type of blood cancer that can cause facial flushing due to increased red blood cell mass and viscosity, leading to increased blood flow to the skin.
Carcinoid Syndrome: A condition caused by a rare type of tumor that secretes serotonin, leading to flushing, diarrhea, wheezing, and heart valve abnormalities.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension, sweating, tachycardia, and flushing. Although rare, missing this diagnosis can be life-threatening due to the risk of hypertensive crises.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, characterized by severe tachycardia, hypertension, fever, and flushing.
Medication-Induced Flushing: Certain medications, such as niacin, calcium channel blockers, and ACE inhibitors, can cause facial flushing as a side effect.

Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, which can also cause facial flushing.
Harlequin Syndrome: A rare condition that affects the nerves that control blood flow to the skin, leading to asymmetric facial flushing and sweating.
Autonomic Dysfunction: Certain conditions, such as multiple system atrophy or pure autonomic failure, can cause abnormal blood flow regulation, leading to facial flushing and other autonomic symptoms.