Ventricular Tachycardia (VT) due to Ischemic Heart Disease: This is the most likely diagnosis given the classic monomorphic appearance of the ventricular tachycardia, which is often associated with a fixed, ischemic substrate such as a prior myocardial infarction.

Idiopathic Ventricular Tachycardia: This could be a consideration, especially if there's no clear evidence of structural heart disease. Idiopathic VT often has a more benign course and can be related to focal areas of abnormal automaticity or reentry.
Right Ventricular Outflow Tract (RVOT) Tachycardia: Although typically seen with a left bundle branch block (LBBB) morphology, some forms of VT originating from the RVOT can present with a monomorphic pattern and might be considered in the differential, especially in younger patients without structural heart disease.

Torsades de Pointes: Although typically polymorphic, in some cases, Torsades can present with a monomorphic pattern, especially if the QT interval is significantly prolonged. Missing this diagnosis could be fatal due to the risk of degenerating into ventricular fibrillation.
Supraventricular Tachycardia (SVT) with Aberrancy: It's crucial to differentiate VT from SVT with aberrancy, as the management and prognosis are significantly different. SVT with aberrancy can mimic VT but requires different treatment approaches.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): This condition can lead to VT but is less common and typically presents with a polymorphic or bidirectional VT pattern. However, it's a rare cause of monomorphic VT and should be considered in patients with a family history or specific structural changes in the right ventricle.
Cardiac Sarcoidosis: This can cause VT due to granulomatous infiltration of the myocardium, leading to scar formation and reentrant arrhythmias. It's a rare but important consideration, especially in younger patients without a clear ischemic cause.