Vasa previa is a rare obstetric complication.
In order for this complication to occur, there has to first be an abnormality of which of the following structures of pregnancy?

The newborn most likely has Brachial plexus injury (B) as evidenced by Left arm remains at the side during Moro reflex (B).

Choice A rationale:

Congenital cardiac defect is less likely since the newborn has a normal heart rate and oxygen saturation. There are no signs of respiratory distress beyond mild acrocyanosis, which is common in newborns and usually resolves without intervention.

Choice B rationale:

Brachial plexus injury is suggested by the newborn's inability to move the left arm during the Moro reflex. This type of injury is often associated with shoulder dystocia and vacuum-assisted delivery, which can stretch or damage the brachial plexus nerves.

Choice C rationale:

Fractured clavicle could be a possibility due to the shoulder dystocia, but it is less likely given that the only noted abnormal finding is the lack of movement in the left arm. Typically, a fractured clavicle would present with crepitus, swelling, or a palpable fracture.

Choice D rationale:

Neonatal sepsis is less likely as the newborn appears to be active, has a normal temperature, and no signs of systemic infection. The only abnormal findings are related to the left arm movement and caput succedaneum, which are more consistent with a birth injury rather than an infection.

Choice A rationale

Thromboembolism is a postpartum complication, particularly in women with risk factors such as obesity, immobility, or a history of thromboembolic events. However, it is not the primary risk associated with a 4,100-gram infant and vacuum-assisted delivery.

Choice B rationale

Postpartum depression is a significant concern for new mothers and can impact their well-being and ability to care for their newborn. While it is a relevant postpartum complication, it is not directly associated with the size of the infant or the delivery method.

Choice C rationale

Mastitis is a potential postpartum complication related to breastfeeding. It involves infection and inflammation of the breast tissue but is not specifically linked to the size of the infant or the mode of delivery.

Choice D rationale

Postpartum hemorrhage is the most significant risk for a client who delivered a 4,100-gram infant via vacuum-assisted delivery. The combination of a large infant and instrumental delivery increases the risk of uterine atony and trauma, leading to excessive bleeding.