To prevent possible retinopathy in a preterm infant requiring oxygen therapy, the nurse will:

Administer low concentrations of oxygen. Retinopathy of prematurity (ROP) is a disease of retinal vascular and capillary proliferation affecting premature infants undergoing oxygen therapy. Oxygen treatment results in pathologic growth of vessels in the developing retina that may lead to permanent damage to the retina as well as retinal detachment and macular folds. Administering low concentrations of oxygen can help prevent ROP by reducing the oxygen-induced vasoconstriction and vascular endothelial growth factor (VEGF) expression.

Choice A is not correct because keeping the infant's eyes covered at all times does not prevent ROP. In fact, it may increase the risk of infection or injury to the eyes.

Choice B is not correct because positioning with the head slightly lower than the body does not prevent ROP. It may increase the intracranial pressure and affect the cerebral blood flow.

Choice D is not correct because monitoring arterial oxygen levels with a pulse oximeter does not prevent ROP. It is a useful tool to guide oxygen therapy, but it does not directly affect retinal vascular development.

Asymmetrical chest movement is a sign of respiratory distress in the newborn, as it indicates unequal lung expansion or airway obstruction. A respiratory rate of 50 breaths/minute (choice B) is normal for a newborn, as is acrocyanosis (choice C), which is a bluish discoloration of the hands and feet due to immature peripheral circulation. Short periods of apnea (less than 15 seconds) (choice D) are also common and benign in newborns unless they are associated with bradycardia or cyanosis.

Choice B is not correct because a respiratory rate of 50 breaths/minute is within the normal range for a newborn.

Choice C is not correct because acrocyanosis is a normal finding in newborns and does not indicate respiratory distress.

Choice D is not correct because short periods of apnea (less than 15 seconds) are normal in newborns and do not indicate respiratory distress.