By now you have probably figured out that veterinary medicine is a combination of medical science and detective work!   With experience, your ability to diagnose based on clinical signs, presenting complaints, and owner information alone will improve.  

But don't be fooled, Vet Student!  Below are two examples of medical conditions that can be misdiagnosed if diagnostic work-ups are skipped and the "likely choices" are assumed!

Scenario One

Female spayed young adult dog presents with owner complaint of bloody urine of three days duration.  Per owner, the dog is acting fine, and is eating and drinking well.  On exam you find no weight loss and no fever.

Diagnostic test chosen:  Urinalysis
Results: TNTC erythrocytes, 1+ WBC, bacteria present
Presumptive diagnosis:  UTI
Treatment: antibiotics

Don't Be Fooled:  Despite owner's report of the dog otherwise acting fine, you notice it seems a bit lethargic but because the dog was otherwise happy and interactive, you ignored it.  You didn't do an oral exam, after all this is a urinary case, so you didn't notice the bleeding gums.  You didn't catch the petechial hemorrhages on the pinnas.  And, you didn't ask enough questions to learn the dog regularly eats mice that it catches, which are in great supply due to the restaurant across the street. 

What you missed:  Anticoagulant Rodenticide Toxicity

Scenario Two

Diagnostic tests chosen:  Urinalysis and CBC/Chem are chosen because you are concerned about potential pancreatitis or diabetes in addition to a probable UTI
Results:  Urine is isosthenuric with heavy WBCs and bacteria.  BUN and Creatinine are slightly elevated. Total WBC is slightly elevated due to neutrophilia.  
Presumptive diagnosis:  Kidney Infection
Treatment:  Antibiotics, subcutaneous fluids, recheck scheduled

Don't be Fooled:  Weight gain and difficult abdominal palpation were probably worth investigating further. But, because you were confident in your diagnosis, you elect not to recommend abdominal radiographs deeming them unneccessary and assuming they would be normal.   What you didn't realize was the extent to which the language barrier impacted the accuracy of the information you received from the owners.

Don't be Fooled!  Clinical experience does not replace the value of diagnostic testing!  As a veterinarian, it is always best to recommend thorough diagnostics, and not assume owners will decline, or results will be normal.