Medical students and recently graduated doctors frequently face difficulties in working out a reasonable differential diagnosis.
In fact the art of making differential diagnoses is largely acquired by experience mixed with adequate knowledge.
The lack of experience is perhaps a reason behind what new doctors sometimes don't hit the correct diagnosis in many clinical situations.
Though this is a real reason, but in medical practice, we are (or should be) adherent to the ABCs of medicine : history taking, physical examination, and work-up.
Detailed and proper history taking is the major contributor of making an exact diagnosis or at least a narrow differential diagnosis.
History taking, by far, remains the cornerstone of medical practice.
The success or failure in reaching a clear conclusion in a given clinical case, is highly dependent on history taking.
I always tell my students “to concentrate on every word spoken by your patient. Every symptom a patient tells, has an explanation. If there is no explanation for a symptom, then there must be an explanation for that”
The patient can really, and unintentionally tell you the diagnosis in an encrypted language composed of symptoms, events ...etc, if well analyzed, it will give you a meaningful medical language.
Of course, you can't rely just on history alone. A careful clinical examination is essential. Even though, clinical examination as well as lab investigations/images, are occasionally dependent on, and for sure, guided by history.
There are many examples where a proper history may save life, and a short , defective history taking may actually kill!
Some diagnoses can be made by a phone call (don't attempt this), with precision, though some are never made by what ever available of knowledge and technology.
I can say that all samples extracted from the patient (tissues or fluids) to be analyzed in the lab, there are equipments used for this purpose.
Till this moment, the words extracted from a patient are only analyzed by the doctor's mind.
Until the time of “history taking machine” comes, we are obliged to adhere to the tradition of good history taking.
So this is no longer a secret! If you want to be exact, listen to your patient and convert the encrypted language into a DIAGNOSIS.