Final answer:
The main problem that should be the focus of nursing interventions for a patient with rheumatic fever suffering from joint discomfort is activity intolerance, which prevents them from engaging in daily activities.
Step-by-step explanation:
A patient with rheumatic fever is experiencing subcutaneous nodules, erythema marginatum, polyarthritis, and joint discomfort that hinders daily activities. The focus of nursing interventions should be on activity intolerance. This condition is due to inflammation that can lead to joint pain and swelling, which is reversible, but if left unchecked, heart valve damage from rheumatic fever can worsen. Joint discomfort, specifically, is preventing the patient from engaging in activities such as walking and sewing. Nursing interventions should aim to manage the patient's symptoms to improve their ability to participate in these activities and enhance their quality of life.