Vasovagal Syncope: This is the most common cause of syncope in young adults, often triggered by stress, pain, or prolonged standing. The patient's symptoms of dizziness and syncope while in the bathroom, along with a relatively normal blood pressure and heart rate, support this diagnosis. The warm extremities also suggest adequate peripheral circulation, which is consistent with vasovagal syncope.

Dehydration: Although the patient has been taking prednisone, which can cause fluid retention, dehydration could still be a factor, especially if she has not been drinking enough fluids. Dehydration can lead to dizziness and syncope.
Orthostatic Hypotension: This condition is characterized by a significant drop in blood pressure upon standing, which can cause dizziness and fainting. The patient's blood pressure is on the lower side, and orthostatic changes might not be fully captured by a single reading.
Anxiety or Panic Attack: These can cause symptoms of dizziness and fainting, especially in a young adult. The stress of the situation or underlying anxiety could contribute to her symptoms.

Cardiac Arrhythmias: Although less common in young adults without a history of heart disease, arrhythmias such as long QT syndrome or other conduction abnormalities could lead to syncope. It's crucial to consider and rule out these potentially life-threatening conditions.
Pulmonary Embolism: Although rare in this age group without specific risk factors, a pulmonary embolism could cause syncope and must be considered, especially if there are other symptoms such as chest pain or shortness of breath.
Hypoglycemia: Low blood sugar can cause dizziness, confusion, and fainting. Given the patient's recent prednisone use, which can affect glucose metabolism, hypoglycemia is a possibility that should not be overlooked.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of high blood pressure, tachycardia, and fainting due to catecholamine surges. The patient's recent use of prednisone might mask some symptoms, but this diagnosis is less likely without other supporting signs.
Subclavian Steal Syndrome: A rare condition where there is a significant stenosis or occlusion of the subclavian artery, leading to syncope due to decreased blood flow to the brain. This would be unusual in a young adult without a history of vascular disease.
Neurocardiogenic Syncope due to Other Causes: Other rare causes such as aortic stenosis, cardiac tamponade, or severe anemia could also lead to syncope but would typically present with additional symptoms or signs.