This paper reviews the progress made towards the development of an Intelligent Computer Assisted Instructional tool designed to function in a medical education setting. The tool, called KBIT (Knowledge Base Inference Tool) is an expert system-based instrument principally consisting of an assessment and a tutorial module. kbit's sole purpose is to support the development and refinement of the differential diagnostic (DDX) knowledge and skills of medical students. The objective of the assessment module is to provide psychometrically reliable and valid measures of several DDS skills. The objective of the tutorial module is to create a learning environment wherein students make refinements in knowledge base (KB) constructs which result in progress towards the next level of DDX skills. kbit's proposed educational approach is comprised of an iterative two-step process consisting of the assessment of several DDX skill performance parameters, followed by individualized formative instruction.