Final answer:
Marcel Breuer is the modern interior designer who famously used tubular steel in his furniture designs, embodying the Bauhaus principles of simplicity and functionality.
Step-by-step explanation:
The modern interior designer known for incorporating tubular steel into furniture designs is Marcel Breuer. Recognized for his innovative use of materials, Breuer designed pieces such as the iconic chair that can be seen on MoMA PDUS. His design is a reflection of the modernist values that were taught and developed at Bauhaus, where he was a student and later a teacher. The Bauhaus movement emphasized that 'less is more' and encouraged the marriage of design and craftsmanship, principles that are evident in the clean, functional forms of Breuer's steel furniture.