Anxiety or Stress: This condition often leads to sinus tachycardia. Left atrial enlargement can be secondary to various conditions, but when combined with sinus tachycardia and incomplete right bundle branch block, it might suggest a condition that causes increased heart rate and potentially increased pressure or volume in the left atrium. Incomplete right bundle branch block can be a benign finding but, in this context, might be related to the overall cardiac stress or condition.

Heart Failure: This can cause left atrial enlargement due to increased pressure and volume overload. Sinus tachycardia can be a compensatory mechanism to increase cardiac output. Incomplete right bundle branch block might be seen in the context of heart disease.
Chronic Obstructive Pulmonary Disease (COPD) or Pulmonary Embolism: Both conditions can lead to tachycardia and may have associated cardiac findings like incomplete right bundle branch block due to increased pulmonary pressures or direct effects on the heart.
Dehydration or Electrolyte Imbalance: These can cause tachycardia. While less directly linked to left atrial enlargement, severe dehydration can lead to a decrease in blood volume, potentially affecting cardiac chamber sizes and function.

Pulmonary Embolism: Although it might not be the first consideration, it's crucial not to miss this diagnosis due to its high mortality rate. Tachycardia and incomplete right bundle branch block can be seen in pulmonary embolism, especially if it's large and causing significant strain on the right heart.
Cardiac Tamponade: This is a life-threatening condition that can cause tachycardia and might lead to chamber enlargement due to impaired cardiac filling. The incomplete right bundle branch block could be a nonspecific finding in the context of acute cardiac stress.
Hypertrophic Cardiomyopathy: This condition can lead to left atrial enlargement and may cause tachycardia. Incomplete right bundle branch block can be seen due to the abnormal cardiac structure.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): This rare condition affects the right ventricle and can lead to arrhythmias and structural changes. While it primarily affects the right ventricle, it could potentially lead to secondary changes in other cardiac chambers.
Cardiac Sarcoidosis: This condition can cause a variety of cardiac abnormalities, including arrhythmias, heart block, and chamber enlargement. It's a rare diagnosis but should be considered in the appropriate clinical context.