Once the definite conclusions have been drawn, the HDP looks for findings and nodes that need explanations and considers causal pathways that might explain them. In the example, there are four nodes with known states. The MI node is true over a time interval, but it is primary (in this limited knowledge base) and needs no explanation. Similarly, the nitroglycerin is a therapy and needs no explanation. The high LAP node is known to be false now, which is its normal state, so no explanation is necessary. The only node that needs an explanation is the pulmonary congestion (PC) node.
The HDP uses the unexplained nodes to generate a list of possible hypotheses. In this case the two primary nodes that could cause PC are MI and pneumonia. The pneumonia causes PC directly and the MI causes it through the causal chain of low LVF and high LAP. The HDP tries to generate each of these hypotheses in turn.