Rheumatic Heart Disease: This is the most common cause of mitral stenosis worldwide, especially in developing countries like Kenya. Rheumatic heart disease often results from untreated or recurrent episodes of rheumatic fever, which can lead to valvular damage, including mitral stenosis.

Congenital Mitral Stenosis: Although less common, congenital abnormalities of the mitral valve can cause stenosis. This condition might be identified during childhood, but mild cases could remain asymptomatic until later in life.
Infective Endocarditis: This condition, while more commonly associated with regurgitant lesions, can occasionally cause stenosis, especially if the infection leads to significant valvular destruction and subsequent scarring.

Marantic Endocarditis (Nonbacterial Thrombotic Endocarditis): Although rare, this condition can cause valvular dysfunction, including stenosis, and is associated with hypercoagulable states or malignancy. Missing this diagnosis could lead to significant morbidity or mortality.
Systemic Lupus Erythematosus (SLE) with Libman-Sacks Endocarditis: SLE can cause nonbacterial endocarditis, leading to valvular lesions, including stenosis. This diagnosis is critical due to the need for immunosuppressive treatment.

Mitral Annular Calcification: This condition is more common in older adults but can occasionally be seen in younger individuals, especially those with certain metabolic disorders. It can lead to mitral stenosis but is less likely in a 21-year-old without other risk factors.
Cardiac Tumors (e.g., Left Atrial Myxoma): Although rare, cardiac tumors can obstruct blood flow and mimic mitral stenosis. Identifying a tumor is crucial for appropriate surgical intervention.
Radiation-Induced Valvular Disease: This would be extremely rare in a 21-year-old unless they had a history of chest radiation for cancer treatment at a young age. Radiation can lead to valvular fibrosis and stenosis over time.