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Meet our Pediatricians

Meet our team


At Cook Children's, taking care of kids means including the whole family. We take the time to listen to you and your child, answer questions and help you navigate parenthood.

Our Keller doctors are specialists in the health care of infants, children and adolescents. Our team includes board certified pediatricians that, together with you, will get your kids on the path to leading happy and healthy lives.

Meet our nurse practitioners

Mercedes Day

Mercedes Day, DNP, APRN, FNP-C

Mercedes Day is a Board Certified Family Nurse Practitioner with an extensive background in pediatrics. She earned her bachelor's degree at Midwestern State University in Wichita Falls, Texas and both a master's and a doctorate degree at Texas Tech University Health Sciences Center in Lubbock, Texas. Mercedes brings a wealth of knowledge and expertise to Cook Children's Keller Heritage Trace primary care office. She has provided quality care to pediatric patients in both primary and urgent care settings. She spent five years as a pediatric nurse in the intensive care unit prior to advancing her career. Mercedes has a passion for education and spends some of her time as an instructor for a master's of nursing program.

Mercedes and her husband Matt have a daughter, ScoutLynn and 2 sons, Kasseus and Atticus who keep her active. Her family enjoys being together and spending time outdoors swimming, skiing and traveling. Mercedes can often be found on the sidelines of her children's sporting events cheering them on as their number one fan.

Staci Lopez-Vergara

Staci Lopez-Vergara, APRN, CPNP

Staci earned her bachelor’s and master’s degree in science and nursing from Texas Tech University Health Sciences Center in Lubbock, Texas. She's board certified through the Pediatric Nursing Certification Board (PNCB) and a member of the National Association of Pediatric Nurse Practitioners (NAPNAP). Her career in pediatric nursing began in 2006 as she cared for medically fragile children in their home and then in the medical/surgical hospital setting. After becoming a pediatric nurse practitioner in 2012, she served children and their families at a large pediatric practice in San Antonio, Texas. In 2014, Staci joined the Cook Children’s family at the Heritage Trace primary care location.

She is passionate about the health and safety of children as well as providing educational experiences for parents to meet their child’s healthcare needs. When Staci is not in the office she enjoys yoga, biking, travel and spending time with her husband, son and their dog Bentley.