Ans. b (Vaginal delivery). (Ref. Harrison's Principles of Internal Medicine, 16th/pg.38, 1082)MATERN AL-FETAL/INFANT TRANSMISSION# The predominant cause of HIV infection in children is transmission of the virus from the mother to the newborn during the perinatal period.# The majority of cases of mother-to-child (vertical) transmission of HIV-1 occur during the intrapartum period.# Exposures, which increase the risk of mother-to-child transmission, include -- Vaginal delivery,- Preterm delivery,- Maternal bleeding, and# Trauma to the fetal skin.# Perinatal HIV transmission can be most accurately correlated with measurement of maternal plasma HIV RNA burden.# In the absence of prophylactic antiretroviral therapy to the mother during pregnancy, labor, and delivery, and to the fetus following birth (see below), the probability of transmission of HIV from mother to infant/fetus ranges from 15- 25% in industrialized countries and from 25-35% in developing countries.# In developed countries, current recommendations to reduce perinatal transmission of HIV include- universal voluntary HIV testing and counseling of pregnant women,- antiretroviral prophylaxis with one or more drugs in cases in which the mother does not require therapy for her HIV infection,- combination therapy for women who do require therapy,- obstetric management that attempts to minimize exposure of the infant to maternal blood and genital secretions, and- avoidance of breast-feeding.# Certain studies have demonstrated that truncated regimens of zidovudine alone or in combination with lamivudine given to the mother during the last few weeks of pregnancy or even only during labor and delivery, and to the infant for a week or less, significantly reduced transmission to the infant compared to placebo.# Short-course prophylactic antiretroviral (ARV) regimens, such as a single dose of nevirapine given to the mother at the onset of labor and a single dose to the infant within 72 h of birth, are of particular relevance to low- to mid-income nations because of the low cost and the fact that in these regions perinatal care is often not available and pregnant women are often seen by a health care provider for the first time at or near the time of delivery.# Indeed, short-course ARV regimens have now been used for several years in developing nations for the prevention of mother-to-child transmission.