Redefining Modern Medicine, One Patient at a Time
At United Medical Doctors (UMD), better healthcare starts with a commitment to people—patients, physicians, and the community.
With more than 70 locations and over 40 specialties, UMD is one of Southern California’s most expansive and innovative physician-led healthcare organizations. Their patient-first model integrates cutting-edge technology, personalized treatment plans, and seamless communication between medical teams—all within a structure designed by doctors, for doctors.
“United Medical Doctors is proud to be physician-run and owned, empowering women in leadership,” says the UMD leadership team.
This physician-led model ensures that clinical decisions prioritize compassion, expertise, and transparency while promoting leadership diversity throughout the organization.
Comprehensive Care in Every Specialty
United Medical Doctors brings a wide array of specialties under one umbrella, making quality care more accessible and coordinated for patients, including:
- Gastroenterology
- Hepatology
- Cardiology
- General and interventional medicine
- Pain management
- Dermatology & Laser Center
- Women’s Health and Minimally Invasive Gynecologic Surgery
- Foot and Ankle Surgical Specialists
- Vein Specialists
- Imaging
By centralizing care through collaborative specialty teams, UMD improves both efficiency and patient outcomes.
Meet the Doctors Making a Difference
Marco A. Paez, MD – Interventional & General Gastroenterology, Hepatology
Dr. Paez is passionate about all aspects of Gastroenterology and Hepatology. He attended Miami-Dade College, Florida International University, and Georgetown University School of Medicine in Washington, D.C., where he graduated with his MD degree in 2012. He then completed his residency in Internal Medicine at Loyola University Medical Center and his general Gastroenterology fellowship at Howard University Hospital. He has a particular clinical interest in Interventional Gastroenterology and completed his advanced fellowship training in Interventional Endoscopy and Endosonography at Harbor-UCLA Medical Center in 2019.
He and his family have now set roots in the Yorba Linda community, and he is excited to advance the quality of GI care in the beautiful city of Yorba Linda.
Scott H. Lee, MD – Gastroenterology, Hepatology
Dr. Scott Lee is a board-certified gastroenterologist who earned his medical degree and completed his training at Loma Linda University. He has practiced in Southern California since 2017, providing comprehensive care to the Inland Empire. Dr. Lee emphasizes a whole-person approach and compassionate care. He enjoys family time, cooking, golf, running, and cheering on Baltimore sports teams.
Christina Steinhauser, DO – Gastroenterology, Hepatology
Dr. Steinhauser is a former Chief of Medicine and Chief of Gastroenterology at the Nellis Air Force Base, as well as a current assistant professor for Uniformed Services University of Health Sciences and a former assistant professor University of Nevada at Las Vegas. As an active-duty physician in the US Air Force for 11 years before joining UMD, she was awarded multiple meritorious awards, including a joint Army-Navy service award and an Army commendation medal while deployed in Germany.
Sumit Khandhar, DO – Cardiology & Vein Specialist
Dr. Khandhar earned his bachelor’s degree from the University of California, Berkeley, and his medical degree from Western University College of Osteopathic Medicine of the Pacific in Pomona, CA. He subsequently completed his Cardiology & Interventional Cardiology Fellowships at Kaiser Permanente Los Angeles Medical Center.
Dr. Khandhar specializes and enjoys caring for patients with various cardiac conditions using a multimodal approach, including preventative and therapeutic methods. He is trained and skilled in performing angiography, angioplasty, and stent placement in simple and complex coronary anatomy.
From Solo Practice to Statewide Leader
UMD’s story began more than 25 years ago when Dr. Thomas Eastman opened a private gastroenterology practice in Fallbrook and Temecula. In 2002, Dr. John Hong joined, and the two physicians launched United Gastroenterologists—a group centered on delivering high-quality, compassionate care.
“From its inception, United Gastroenterologists has been committed to providing the highest level of quality care,” says Dr. Hong, MD, CEO, MBA.
In 2017, the group evolved into United Medical Doctors, expanding from a single-specialty group into a leading multi-specialty network serving patients across Southern California.
More Than a Medical Group
Beyond its specialty services, UMD operates a growing network of United Surgery Centers (USC), offering outpatient surgical procedures in a convenient, high-tech setting. The group is also deeply involved in clinical research and healthcare education through United Nursing College—training the next generation of medical professionals.
UMD is proud to be a member of the UC San Diego Health Physician Network and was among the inaugural partners of the UCI Health Clinically Integrated Network (CIN) —a collaborative initiative aimed at improving health outcomes, increasing access, and reducing costs through coordinated care.
Committed to Community Health
Whether you need a cardiologist in Brea, GI specialist in Yorba Linda, or outpatient surgery, UMD is committed to serving your health with compassion and expertise. To schedule an appointment with the UMD team in Yorba Linda, call (657) 866-3017, or in Brea, call (657) 286-2670. For additional information, visit our website - unitedmd.com.
“By fostering both clinical autonomy and leadership diversity, United Medical Doctors creates an environment where exceptional care and professional excellence thrive together.” —UMD
"Driven by compassion and clinical excellence, UMD empowers doctors to lead, innovate, and deliver truly personalized patient care every day."