Differential diagnosis

Differential diagnosis is detective work: the client's symptoms could fit several diagnoses, and your job is to systematically rule out the alternatives until the best-fitting explanation remains. Examiners test whether you reason carefully — especially whether you remember to check for medical causes before landing on a psychiatric label.

The core move is generating a differential — a list of candidate diagnoses that share the presenting symptoms — and then ruling out competitors using criteria, course, and context. A cornerstone rule: rule out medical conditions and substance effects first. Many psychiatric-looking presentations are actually medical mimics — for example, thyroid dysfunction can masquerade as depression or anxiety, delirium can look like a psychotic or mood episode, vitamin deficiencies and neurological conditions can mimic cognitive disorders, and substance intoxication or withdrawal can imitate almost anything. This is why the DSM builds in "due to another medical condition" and substance/medication-induced categories: you must exclude those before diagnosing a primary mental disorder. When in doubt, refer for a medical evaluation.

Then there's comorbidity thinking — the recognition that clients frequently meet criteria for more than one disorder at the same time (comorbidity is the norm, not the exception). Differential diagnosis isn't always about choosing a single winner; sometimes the accurate picture is two co-occurring conditions. The skill is holding both possibilities — "which one?" and "could it be both?" — without prematurely collapsing to one label.

Clinical Pearl: The exam's favorite differential-diagnosis reflex: rule out medical and substance-induced causes before assigning a psychiatric diagnosis. If a stem shows depressive or anxious symptoms with a possible physical cause (thyroid, medication, substance), the right first step is a medical evaluation / rule-out, not an immediate mental-disorder label.