Side-by-side tables of the look-alike diagnoses the Prometric loves to confuse, each with the one discriminator the exam uses.
| Feature | DKA | HHS |
|---|---|---|
| Typical patient | Type 1, younger; also SGLT2i users (euglycaemic DKA) | Older type 2, often infection or poor intake |
| Onset | Hours to 1 day | Days to weeks |
| Glucose | > 250 mg/dL classic (ADA 2024: ≥ 200 or known diabetes) | ≥ 600 mg/dL (33 mmol/L) |
| pH / HCO3 | pH < 7.30, HCO3 < 18 | pH ≥ 7.30, HCO3 ≥ 15 (older criteria > 18) |
| Ketones | β-hydroxybutyrate ≥ 3 mmol/L | Absent or minimal |
| Osmolality / AG | Variable osm; high anion gap | Total osm > 320; AG variable |
| Mental status | Alert to drowsy | Confusion / coma common |
| Treatment | Saline, IV insulin 0.1 U/kg/h, K replacement; add dextrose when glucose ~250 and keep insulin until AG closes | Saline first (bigger deficit); insulin low dose once glucose stops falling with fluids; lower osm and Na slowly |
Potassium rule: K < 3.3 → hold insulin and replace K first; 3.3–5.2 → add K to fluids; > 5.2 → no K, recheck (2024 consensus: hold insulin if K < 3.5).
| Feature | Ulcerative colitis | Crohn's disease |
|---|---|---|
| Site | Rectum, extends proximally, continuous | Mouth to anus, skip lesions; terminal ileum commonest |
| Depth | Mucosal | Transmural |
| Key clue | Bloody diarrhoea, tenesmus | Pain RLQ, weight loss, perianal disease, fistulae, strictures |
| Histology | Crypt abscesses, no granulomas | Non-caseating granulomas |
| Serology | p-ANCA | ASCA |
| Imaging | Lead-pipe colon (loss of haustra) | String sign, cobblestoning, skip lesions |
| Smoking | Protective | Worsens disease |
| Associations | PSC, toxic megacolon, colon cancer | Gallstones, oxalate kidney stones, B12 deficiency |
| Surgery | Colectomy is curative | Not curative; recurs at anastomosis |
| Feature | SIADH | Cerebral salt wasting | Central DI | Nephrogenic DI |
|---|---|---|---|---|
| Volume | Euvolaemic | Hypovolaemic | Normal / dry | Normal / dry |
| Serum Na | Low | Low | High-normal / high | High-normal / high |
| Urine osm | > 100 (inappropriately concentrated) | High | Low (dilute) | Low (dilute) |
| Urine Na | > 30 mmol/L | > 30 (very high) | Low / variable | Low / variable |
| Urine output | Normal / low | High | High | High |
| Desmopressin | — | — | Urine osm rises clearly | Little or no rise |
| Causes | Small cell lung cancer, CNS, pneumonia, SSRIs, carbamazepine | SAH, neurosurgery, head injury | Pituitary surgery, trauma, tumour | Lithium, hypercalcaemia, hypokalaemia |
| Treatment | Fluid restriction; hypertonic saline if severe; tolvaptan | Isotonic saline ± fludrocortisone | Desmopressin | Stop cause; thiazide, amiloride (lithium), NSAID |