Advanced Differential Diagnosis

1. Build the list wide before you narrow it

A field differential is a list of plausible explanations you hold at the same time. Not one at a time. You're not picking your favorite and testing it alone. You're carrying three or four candidates in your head together, and you're looking for the finding that knocks one of them out.

For a presentation that doesn't localize, you build breadth first. Altered mental status is the clearest example. AEIOU-TIPS is the named structure for it:

Letter	Covers
A	Alcohol, acidosis
E	Epilepsy (postictal), electrolytes, encephalopathy, environmental
I	Insulin — hypoglycemia and hyperglycemia
O	Overdose, oxygen (hypoxia), opiates
U	Uremia and other metabolic or renal causes
T	Trauma, temperature (hypo- and hyperthermia)
I	Infection — including sepsis and meningitis
P	Psychiatric, poisoning
S	Stroke, shock, space-occupying lesion, subarachnoid hemorrhage

You're not reciting it for a coworker. You're using it to check your own coverage. Does your list actually span metabolic, toxicologic, neurologic, infectious, traumatic, environmental, and psychiatric causes? Or did you stop at the first plausible one?

That same discipline runs through every undifferentiated presentation, not just altered mental status.

Undifferentiated shock. Is it hypovolemic, cardiogenic, obstructive, or distributive? Obstructive covers tension pneumothorax, tamponade, and massive PE. Distributive covers septic, anaphylactic, and neurogenic. Each category has a different field lever. Pick the wrong one and you give fluid to a patient who needed decompression instead.
Chest pain. ACS, PE, aortic dissection, tension pneumothorax, pericarditis, esophageal rupture, pneumonia, and musculoskeletal causes are all on the list until something rules them out.
Dyspnea. Is the failure in ventilation, in diffusion, or in perfusion? Those are three different problems that all show up as "can't breathe."
Abdominal pain. This one trips people up because the list is not all abdominal. AAA, mesenteric ischemia, ectopic pregnancy, and inferior MI all present as abdominal pain, and none of them are abdominal problems.

Here's the part the exam leans on hardest. Secondhand history — "he's just drunk," "she does this all the time," whatever dispatch typed as the chief complaint — should widen your differential. Not narrow it. An unreliable history is a reason to check more possibilities, not fewer. The National Registry writes items where the easy read is to trust the label someone else already put on the patient. That's the trap.