Hypertrophic Cardiomyopathy: This condition is characterized by an abnormal thickening of the heart muscle, which can lead to deep QRS complexes on an electrocardiogram (ECG). The narrow QRS duration suggests that the conduction delay is not significant, pointing more towards a muscular issue rather than a conduction system problem.

Left Ventricular Hypertrophy (LVH): LVH can result from various conditions such as hypertension, aortic stenosis, or other heart diseases, leading to increased QRS complex amplitude due to the thicker ventricular wall.
Athlete's Heart: Athletes, especially those involved in endurance sports, can develop physiological cardiac adaptations including increased left ventricular wall thickness, which might result in deeper QRS complexes on ECG.

Hyperkalemia: Although less common in a young individual without known renal issues, hyperkalemia can cause tall, peaked T waves and can also affect QRS complex morphology, making it wider rather than narrower. However, early signs might include subtle changes in QRS morphology.
Cardiac Sarcoidosis: This condition can lead to various ECG abnormalities, including conduction disturbances. While it might not typically cause deep narrow QRS complexes, its potential for serious outcomes, including heart block and sudden death, makes it a "do not miss" diagnosis.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): This rare condition involves the replacement of myocardial tissue by fibrofatty tissue, primarily in the right ventricle. It can lead to various ECG abnormalities, including those affecting the QRS complex, but is less likely to present with deep narrow QRS complexes as a primary finding.
Cardiac Fabry Disease: A genetic disorder that can lead to left ventricular hypertrophy among other cardiac manifestations. While it's a rare condition, it could potentially cause deep QRS complexes due to the hypertrophy, but it's less common and typically associated with other systemic symptoms.