An 18-year-old young man complains of pain in his knee and ankle joints and a fever of 39.5°C. A week and a half before, he had a case of respiratory disease. His body temperature is 38.5°C, his knee and ankle joints are swollen. His pulse is 106/min., rhythmic. His blood pressure is 90/60 mm Hg. The heart borders are normal, the heart sounds are weakened, and there is a soft systolic murmur at the apex. What parameter was most closely associated with the possible etiology of this process?