Final answer:
The patient's family history of sudden cardiac arrest is the most important information for planning care with hypertrophic cardiomyopathy, due to the genetic link and increased risk of similar events.
Step-by-step explanation:
The most important information obtained by the nurse from a 24-year-old patient with hypertrophic cardiomyopathy (CMP) in planning care would be the patient's 29-year-old brother dying from sudden cardiac arrest. This piece of medical history is crucial because it suggests a potential genetic link and an increased risk for similar cardiac events. Considering the nature of hypertrophic cardiomyopathy and its connection with sudden cardiac arrests, especially in younger individuals, identifying family history of such occurrences can impact both the urgency and the nature of the care plan including monitoring strategies, lifestyle recommendations, and possibly medical or surgical treatments.