Which of the following conditions is NOT commonly associated with malignancy?

Pemphigus is primarily an autoimmune condition characterized by blistering of the skin and mucous membranes due to the production of antibodies against desmogleins, which are proteins critical for cell adhesion in the skin. While pemphigus can be associated with other conditions, it is not commonly linked with malignancies in the same way that the other conditions listed are.

In contrast, the other conditions mentioned have established associations with certain types of cancers. Acanthosis nigricans, for instance, may manifest as a skin change related to insulin resistance or endocrine abnormalities, but it notably can be a paraneoplastic syndrome linked to gastric adenocarcinoma and other malignancies. Erythema nodosum is often associated with underlying systemic conditions, including malignancy, especially in the context of lymphatic cancers. Dermatomyositis has a well-documented association with various malignancies, particularly in adults, where it can serve as a paraneoplastic syndrome indicative of underlying cancer.

Thus, recognizing pemphigus as a condition that is primarily autoimmune and less frequently associated with malignancies helps to clarify why it is the correct answer in this context.