The location between the anus and the scrotum is called the perineum, and a four-month-old child with redness below the anus and testicles is red for the perineum, which may be caused by diaper rash due to improper use of diapers; in addition, it is necessary to consider the possibility of vulvovaginitis caused by local skin infections. 1. diaper rash: some infants and young children due to wear diapers wrapped too tightly or improper use of the perineum friction caused by redness and swelling. 2. Vulvovaginitis: babies do not change diapers in time, urine, stool stimulation can lead to local skin infections can also cause redness. Overall, if you find the redness of the perineal skin of infants and young children, you should go to the hospital pediatrics examination, clear causes under the guidance of the doctor for targeted treatment.