Congenital syphilis

A 28-month-old boy is brought to the pediatrician for a routine evaluation. The patient was born via vaginal delivery at 38-weeks of gestational age. The patient’s mother received minimal antenatal care. During the third trimester of pregnancy, the mother had developed a disseminated maculopapular rash involving the palms and soles and arthralgias. The patient’s weight is at the 25th percentile, and head circumference is at the 50^th percentile. Vitals are within normal limits. Abdominal examination reveals hepatosplenomegaly. A depressed nasal bridge, malformed teeth, and bowing of the tibia are also observed. Cardiovascular examination is unremarkable. Which of the following is the most likely cause of this patient’s condition?