What does Xiao Ao mean by telling a joke to Shenlong Xiao at the beginning of June 24th?

In the sixties and seventies, some barefoot doctors with little education were "taken care of" and entered the hospital. At that time, the folk customs were simple, so the following true story happened:

One day, a middle-aged woman went to the hospital to see a doctor and happened to meet such a "barefoot doctor". The middle-aged woman reported that her thumb was inflamed. The doctor looked at the patient's thumb and decided to take an X-ray, so he wrote a checklist and said to the patient, "Go to the radiology department!" " The woman looked at the list, paused and turned away without asking any more questions.

The next day, the woman took her son to the radiology department of the hospital and asked for a photo of her mother and son. The radiologist was very surprised and told the patient that the hospital was not a photo studio. But the woman insisted that it was sent by the doctor and took out the checklist. The radiologist opened it and it said "mother-child photography" (thumb photography).