Type 2 Diabetes Mellitus: Given the patient's age, family history of diabetes, and elevated HbA1c level of 7.4%, along with postprandial glucose above 200, this diagnosis is the most likely. Type 2 diabetes is characterized by insulin resistance and impaired insulin secretion, which aligns with the patient's presentation.

Impaired Glucose Tolerance (IGT): Although the HbA1c is above the threshold for diabetes, some individuals may have impaired glucose tolerance, especially if the postprandial glucose levels are not consistently above 200. IGT is a pre-diabetic state that can progress to diabetes.
LADA (Latent Autoimmune Diabetes in Adults): This form of diabetes has an autoimmune component similar to type 1 diabetes but typically develops in adults and may initially be mistaken for type 2 diabetes due to the age of onset. The presence of autoantibodies would support this diagnosis.
Steroid-Induced Diabetes: If the patient is on steroid therapy, this could be a contributing factor to the elevated glucose levels. Steroids can induce insulin resistance.

Type 1 Diabetes: Although less common in adults, type 1 diabetes can occur at any age and is characterized by absolute insulin deficiency. It is crucial to distinguish from type 2 diabetes due to the need for insulin therapy from the outset.
Pancreatic Insufficiency: Conditions such as pancreatitis or pancreatic surgery can lead to diabetes due to the destruction of pancreatic tissue. This would be a critical diagnosis not to miss due to its implications for management and potential for other complications.
Cushing's Syndrome: This rare endocrine disorder can cause diabetes due to excess cortisol production. It's essential to consider due to its significant impact on morbidity and mortality if left untreated.

MODY (Maturity-Onset Diabetes of the Young): This is a form of diabetes caused by genetic mutations that affect insulin production. It typically presents at a young age and can be mistaken for type 1 or type 2 diabetes. Genetic testing can confirm the diagnosis.
Wolfram Syndrome: A rare genetic disorder characterized by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness. It's an important consideration in young patients with diabetes and other systemic symptoms.
Hormonal Imbalances: Certain hormonal imbalances, such as acromegaly or pheochromocytoma, can cause diabetes. These conditions are rare but critical to diagnose due to their potential for significant morbidity and mortality.