Final answer:
Sezary syndrome is characterized by the presence of atypical Cd4+ cells with cerebriform nuclei on a blood smear, marking its progression from mycosis fungoides.
Step-by-step explanation:
The characteristic feature of Sezary syndrome is the presence of atypical Cd4+ cells with cerebriform nuclei seen on a blood smear. This condition is, in fact, a progression of another disease known as mycosis fungoides. Sezary syndrome involves the proliferation of malignant T cells, which manifest in the skin and in the blood, leading to various clinical symptoms including erythroderma, lymphadenopathy, and pruritus.