What is the most appropriate management for a 26-year-old woman at 28 weeks of pregnancy with a history of urinary tract infection on urinalysis?

In the instance of a pregnant woman presenting with urinalysis results indicating a urinary tract infection (UTI), beginning treatment with cephalexin is the most appropriate management strategy. Cephalexin is a first-line antibiotic that is safe for use during pregnancy and is effective against the common pathogens responsible for UTIs.

During pregnancy, the physiological changes can lead to increased susceptibility to UTIs, and immediate management is crucial to prevent complications such as pyelonephritis, which can pose significant risks to both the mother and the fetus. Starting an appropriate antibiotic promptly helps to ensure that the infection is resolved quickly and effectively.

While other management options might be considered in different contexts, they are not as suitable in this situation. For instance, starting antibiotics immediately could suggest using an unspecified antibiotic that may not be optimal for the patient's situation, and referring to a specialist would typically not be necessary unless there are complicating factors such as recurrent infections or other significant medical concerns. Monitoring and retesting in a week would not adequately address an existing infection that requires treatment and could lead to further complications.

Therefore, initiating treatment with cephalexin is the best course of action based on the provided urinalysis findings and the patient's pregnant status.