House's second medical specialty after infectious disease in House M.D. was nephrology. The character is described as a board-certified diagnostician with expertise in both infectious disease and nephrology.
Nephrology is the medical specialty concerned with the kidneys, kidney function, fluid balance, electrolytes, and related disorders. Infectious disease focuses on illnesses caused by infectious organisms and on complex infections. The combination helps explain why House is portrayed as capable of handling a wide range of puzzling cases.
The show usually emphasizes House's diagnostic identity rather than treating him as a conventional specialist. His team may investigate neurological, autoimmune, toxicological, cardiac, or genetic conditions, but House's formal background remains especially associated with infectious disease and nephrology.
Viewers sometimes assume neurology is one of House's specialties because many episodes involve the brain or unusual neurological symptoms. However, the canonical character background identifies nephrology, not neurology, as the second specialty.