In infants who are younger than 3 months old, temperatures greater than 100,4 degrees Fahrenheit may be a sign of

Rationale: A. Metabolic acidosis may cause rapid breathing or lethargy but is not primarily indicated by fever. B. In infants younger than 3 months, a temperature greater than 100.4°F (38°C) is concerning for a serious bacterial or viral infection due to their immature immune systems. Even mild fevers in this age group warrant prompt evaluation for sepsis, urinary tract infections, or meningitis. C. Renal artery disease does not typically present with isolated fever in young infants. D. Congenital heart defects may cause cyanosis, tachypnea, or poor feeding, but fever is not a primary sign.