Essential Hypertension: Although less common in children than in adults, essential hypertension is still a leading cause of high blood pressure and can present suddenly. It is often associated with a family history of hypertension, obesity, and other lifestyle factors.

Renal Disease: Conditions such as glomerulonephritis, renal artery stenosis, and chronic kidney disease can cause sudden onset of high blood pressure in children due to the kidney's role in blood pressure regulation.
Coarctation of the Aorta: A congenital condition characterized by the narrowing of the aortic isthmus, leading to increased blood pressure in the upper body.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension due to the excessive production of catecholamines.
Hyperthyroidism: An overactive thyroid gland can lead to increased blood pressure due to the metabolic effects of excess thyroid hormones.

Hypertensive Crisis due to Medications or Substances: Certain medications, illicit drugs (e.g., cocaine, amphetamines), and substances (e.g., certain over-the-counter medications like decongestants) can cause a sudden and dangerous increase in blood pressure.
Neurological Causes: Conditions such as increased intracranial pressure, brain tumors, or spinal cord injuries can lead to sudden hypertension due to the body's response to stress or direct effects on blood pressure regulatory centers in the brain.
Adrenal Causes: Besides pheochromocytoma, other adrenal disorders like Cushing's syndrome or congenital adrenal hyperplasia can cause hypertension.

Vasculitis: Inflammation of blood vessels, such as in conditions like Kawasaki disease or Takayasu arteritis, can lead to hypertension due to the narrowing or damage of blood vessels.
Sleep Apnea: Although more commonly associated with adults, sleep apnea can also occur in children and contribute to the development of hypertension.
Lead Poisoning: Exposure to lead can cause hypertension among other health issues, particularly in children living in environments with high levels of lead contamination.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, which can present with sudden and severe hypertension among other symptoms.