Bicuspid Aortic Valve: This is the most common congenital heart defect and a frequent cause of aortic stenosis, particularly in patients who develop symptoms at a younger age, such as in their 50s. The bicuspid aortic valve is more prone to early calcification and stenosis compared to a tricuspid valve.

Degenerative (Senile) Calcific Aortic Stenosis: Although this typically presents later in life, it's possible for it to occur in the 50s, especially with risk factors such as hypertension, hypercholesterolemia, or smoking.
Rheumatic Heart Disease: Though less common in developed countries due to the decline of rheumatic fever, it can still cause aortic stenosis, often in conjunction with mitral valve disease.

Aortic Stenosis due to Radiation Therapy: Patients with a history of chest radiation (e.g., for Hodgkin lymphoma) are at increased risk of developing valvular heart disease, including aortic stenosis, and this can occur at any age post-exposure.
Inflammatory Causes (e.g., Lupus, Rheumatoid Arthritis): These conditions can lead to valvular heart disease, including aortic stenosis, and are important to consider due to their potential for other systemic manifestations and the need for specific treatment.

Congenitally Unicuspid or Quadricuspid Aortic Valve: These are rare congenital anomalies of the aortic valve that can lead to aortic stenosis or regurgitation.
Aortic Stenosis due to Infective Endocarditis: Though more commonly associated with aortic regurgitation, infective endocarditis can rarely cause stenosis, particularly if there is significant valvular destruction and fibrosis.
Medications or Toxins (e.g., Anorectic Drugs): Certain medications and toxins have been associated with valvular heart disease, including aortic stenosis, though these are rare causes.