What is the diagnosis for erythematous, ill-defined papules and blisters that dry and crust over, typically found in flexural areas?

The diagnosis of erythematous, ill-defined papules and blisters that dry and crust over, especially in flexural areas, aligns closely with atopic dermatitis. This condition, often seen in children, is characterized by an itchy, inflammatory skin reaction. The appearance of blisters and crusting is indicative of the fluctuating nature of atopic dermatitis, which often presents with acute exacerbations and is common in areas such as the elbows and knees.

Atopic dermatitis is associated with a history of allergy, asthma, or hay fever, and the lesions can become infected, leading to the crusting appearance described. The ill-defined borders of the lesions are also characteristic, differentiating it from other skin conditions that may have more sharply demarcated features.

Other skin conditions listed, while potentially presenting with papules or erythema, have different characteristics or typical presentations that do not match the flexural localization or the episodic, weeping nature of atopic dermatitis.