Four adages I learned in medical training that I still speak of today:

“Common things are common.” (The alternate version of this that might have more appeal to zoologists: “When you hear hoofbeats, think horses, not zebras.”)

This cautions physicians to remember that it is more likely that the patient has a common condition than a rare one. Although it is prudent to consider all the possible diagnoses that might match a given clinical presentation, one should not seek confirmation for an exotic condition first.

Urinary tract infections are more common than bladder cancers. High blood pressure is more common than pheochromocytomas.

Once you’re sure that there are no horses present, though, then begin the search for other ungulates.

“Treat the patient, not the number.” This is a reminder that physicians should treat the person, not lab results.

If a patient’s blood count is a little low, but she’s not experiencing any symptoms, then do nothing. If someone’s lithium level has been low for months, but they haven’t had any mood symptoms, then don’t increase the dose of lithium.

This, however, does not apply to all conditions: People with alarming blood pressure numbers often feel fine. Same thing with high blood sugar numbers.

“The longer someone stays in the hospital, the longer he stays in the hospital.” Hospitals are not sanitary places. The longer a patient stays in the hospital, the more likely he will develop an infection that is resistant to multiple antibiotics. This leads to complications that lengthens the hospital stay.

This also applies to staff: The longer a physician stays in the hospital (beyond her shift, for example), more things will come up that she will have to address, which will will keep her there even longer.

“When you only have a hammer, everything is a nail.” This is a reminder to consider other perspectives. It is also an exhortation to recruit the minds and skills of others.

If the psychiatrist only knows how to prescribe medications, then all of his patients will receive pills. The surgeon might believe that cutting out the offending tissue is the only solution.

One wonders when these phrases first came into being. It’s an oral history that physicians pass along every July.

Maria Yang is a psychiatrist who blogs at her self-titled site, Maria Yang, MD.