The San Diego Zoo Safari Park performed a biopsy on a northern white rhino Thursday. Nola, 41, is one of only four northern white rhinoceroses left in the world.

A Safari Park veterinary team performed the procedure to determine what is causing swelling in Nola’s hip.

Vets treated her back in May for an abscess in the same hip. With antibiotics, the wound seemed to heal, but a week before the procedure it appeared to reemerge along with swelling on her hip.

Nola stood in a barn inside her zoo habitat under mild sedation for the procedure. She received alfalfa and apple treats and head rubs during the procedure.

“Today we performed a biopsy on Nola’s hip, in an attempt to see what might be causing the abscess and the swelling,” stated Dr. Jeff Zuba, senior veterinarian at the San Diego Zoo Safari Park. “At this point, it appears the original abscess sealed itself off without completely healing, but we need to investigate further. We obtained tissue and fluid samples from the hip swelling to assist with the diagnosis. A blood sample also was collected to assess overall health. Until we receive these test results, we will start her on antibiotics based on culture results obtained last week and provide daily wound care, while monitoring her closely.”

She will be kept in the barn overnight and released back into her habitat in the morning. After the procedure she seemed to be feeling well and walking and eating normally. Zoo officials say test results will be back in seven to ten days.

“Nola was an exceptional patient, as usual,” said Zuba.

Northern white rhinos have been hunted to near extinction for their horns, which are believed in some cultures to possess healing power.

The Safari Park is also using science to fight the rhino’s extinction. The Frozen Zoo, in collaboration with the Scripps Research Institute in La Jolla, is a genetic project to help save endangered species.

The zoo has viable stem cells taken from twelve white rhinos to restore genetic variation. The rhino stem cells, first saved in 1979, can be used to reproduce any tissue in the rhino’s body, the zoo says.