USMLE Differential Diagnoses

Side-by-side breakdowns of the conditions the USMLE loves to confuse — the features that truly separate them, the tests that discriminate, and the reasoning to tell them apart.

• Achalasia vs Diffuse Esophageal Spasm: How to Tell Them ApartAchalasia and diffuse esophageal spasm are both esophageal motility disorders causing dysphagia and chest pain. The key axis is LES function and peristaltic pattern, revealed by manometry and barium swallow.
• Acute Cholecystitis vs Ascending Cholangitis: How to Tell Them ApartA high-yield USMLE comparison of acute cholecystitis and ascending cholangitis, centered on the key axis of obstruction level: cystic duct inflammation versus common bile duct obstruction with jaundice.
• Acute vs Chronic Leukemia: How to Tell Them ApartCompare acute leukemia (ALL, AML) with chronic leukemia (CLL, CML) for the USMLE, focusing on the core axis of blast-driven maturation arrest versus indolent accumulation of mature cells.
• Acute vs Chronic Pancreatitis: How to Tell Them ApartDistinguish acute pancreatitis from chronic pancreatitis by course, diagnostic criteria, imaging, and exocrine/endocrine function — a high-yield USMLE comparison of these two pancreatic inflammatory diseases.
• Aortic Stenosis vs Aortic Regurgitation: How to Tell Them ApartCompare aortic stenosis and aortic regurgitation for the USMLE: obstruction versus incomplete closure, systolic versus diastolic murmurs, pressure versus volume overload, and the physical exam findings that separate them.
• Aortic Stenosis vs Mitral Regurgitation: Telling Two Systolic Murmurs ApartA high-yield USMLE comparison of aortic stenosis and mitral regurgitation, focused on the core axis of pressure overload versus volume overload and how each valve lesion, murmur, and pattern of ventricular remodeling differs.
• Asthma vs COPD: How to Tell Them ApartAsthma and COPD both cause airflow obstruction and wheezing, but they differ on the core axis of reversibility. Compare onset, inflammation, bronchodilator response, and treatment for the USMLE.
• ATN vs AIN: How to Tell Them ApartAcute tubular necrosis versus acute interstitial nephritis for the USMLE: distinguish these intrinsic causes of AKI by mechanism, urine sediment, triggers, and clinical clues.
• Atrial Fibrillation vs Atrial Flutter: How to Tell Them ApartAtrial fibrillation and atrial flutter are supraventricular tachyarrhythmias that share management and stroke risk but differ fundamentally in atrial mechanism and ECG appearance. Learn the high-yield discriminators for the USMLE.
• Bacterial vs Viral Meningitis: How to Tell Them ApartCompare bacterial and viral (aseptic) meningitis for the USMLE, focusing on the CSF pattern, illness severity, mental status, and urgency of treatment that separate these two causes of meningeal inflammation.
• Crohn's vs Ulcerative Colitis: How to Tell Them ApartCrohn's disease vs ulcerative colitis — the differences in location, depth, endoscopy, histology, antibodies, and complications, plus the clinical reasoning to distinguish them for the USMLE.
• Cushing Syndrome vs Addison Disease: How to Tell Them ApartCushing syndrome (cortisol excess) versus Addison disease (primary adrenal insufficiency): compare habitus, blood pressure, electrolytes, skin findings, and the cortisol/ACTH testing that separates hormone excess from deficiency.
• Delirium vs Dementia: How to Tell Them ApartDelirium and dementia both cause confusion and cognitive impairment, but they diverge on the core axis of tempo and reversibility—acute fluctuating states versus chronic progressive decline. Learn the high-yield discriminators for the USMLE.
• DKA vs HHS: How to Tell Them ApartDiabetic ketoacidosis versus hyperosmolar hyperglycemic state for the USMLE: distinguishing these two hyperglycemic crises by ketosis, acidosis, glucose, osmolality, and insulin deficiency.
• Emphysema vs Chronic Bronchitis: How to Tell Them ApartCompare emphysema and chronic bronchitis, the two faces of COPD, along the core axis of parenchymal destruction versus airway/mucus disease, with high-yield discriminators, tests, and pitfalls for the USMLE.
• Gastric Ulcer vs Duodenal Ulcer: How to Tell Them ApartGastric ulcer versus duodenal ulcer for the USMLE: distinguish them by pain-food timing, H. pylori infection pattern, acid output, malignancy risk, and management to nail the diagnosis.
• GERD vs Peptic Ulcer Disease: How to Tell Them ApartCompare gastroesophageal reflux disease (GERD) and peptic ulcer disease (PUD) for the USMLE: symptom pattern, underlying mechanism, H. pylori's role, cancer risk site, and the tests that separate acid reflux from mucosal ulceration.
• Gout vs Pseudogout: How to Tell Them ApartCompare gout and pseudogout (CPPD) for the USMLE: crystal morphology and birefringence, joint distribution, radiographic findings, and treatment differences that separate these two crystal arthropathies.
• Hashimoto Thyroiditis vs Graves Disease: How to Tell Them ApartHashimoto thyroiditis and Graves disease are both autoimmune thyroid disorders, but they sit at opposite ends of thyroid function. Learn to separate hypothyroid destruction from hyperthyroid stimulation using TSH, antibodies, RAIU, and eye findings.
• Hypothyroidism vs Hyperthyroidism: How to Tell Them ApartCompare hypothyroidism and hyperthyroidism for the USMLE: the deficiency-versus-excess axis, opposite clinical signs, TSH/Free T4 patterns, antibodies, RAIU, and the emergencies that distinguish them.
• Iron Deficiency Anemia vs Anemia of Chronic DiseaseIron deficiency anemia versus anemia of chronic disease: distinguish these overlapping microcytic anemias using ferritin, TIBC, and iron studies for the USMLE.
• Ischemic vs Hemorrhagic Stroke: How to Tell Them ApartIschemic and hemorrhagic stroke both cause sudden focal neurological deficits, but they diverge on mechanism, imaging, and treatment. Learn how CT, headache, and history separate a clot from a bleed.
• ITP vs TTP: How to Tell Them ApartITP and TTP both cause thrombocytopenia and mucocutaneous bleeding, but only TTP produces microangiopathic hemolytic anemia with schistocytes. Learn the high-yield discriminators for the USMLE.
• Migraine vs Cluster Headache: How to Tell Them ApartMigraine and cluster headache are both severe primary headaches, but they diverge on laterality, autonomic features, duration, and patient behavior. Master the high-yield discriminators for the USMLE.
• Minimal Change Disease vs FSGS: How to Tell Them ApartMinimal change disease vs focal segmental glomerulosclerosis (FSGS): both cause nephrotic syndrome, but they differ in age, hematuria, blood pressure, histology, steroid response, and prognosis.
• Myasthenia Gravis vs Lambert-Eaton Syndrome: How to Tell Them ApartBoth cause fatigable neuromuscular-junction weakness, but myasthenia gravis is a postsynaptic (AChR) disorder while Lambert-Eaton is presynaptic (VGCC). Compare reflexes, facilitation, autonomic signs, antibodies, and cancer associations for the USMLE.
• Nephrotic vs Nephritic Syndrome: How to Tell Them ApartNephrotic syndrome (the leaky glomerulus) versus nephritic syndrome (the inflamed glomerulus): compare proteinuria, hematuria, RBC casts, edema, hypertension, and the prototype diseases for the USMLE.
• Parkinson Disease vs Essential Tremor: How to Tell Them ApartParkinson disease and essential tremor both cause a hand tremor in older adults, but the defining axis is resting versus action tremor plus the presence or absence of bradykinesia, rigidity, and gait impairment.
• Placenta Previa vs Placental Abruption: How to Tell Them ApartPlacenta previa and placental abruption both cause third-trimester vaginal bleeding, but they split along a critical axis: painless bleeding with a soft uterus versus painful bleeding with a rigid, tender uterus and fetal distress.
• Polymyositis vs Dermatomyositis: How to Tell Them ApartPolymyositis and dermatomyositis both cause symmetric proximal muscle weakness with elevated CK, but pathognomonic skin findings, distinct biopsy patterns, and malignancy risk separate them. A high-yield USMLE comparison.
• Preeclampsia vs Eclampsia: How to Tell Them ApartPreeclampsia and eclampsia sit on the same hypertensive-disorder-of-pregnancy spectrum. The defining axis is seizures: eclampsia is preeclampsia complicated by generalized tonic-clonic convulsions. Here is how to distinguish them for the USMLE.
• Prerenal AKI vs Intrinsic AKI (ATN): How to Tell Them ApartPrerenal azotemia versus acute tubular necrosis (ATN) on the USMLE: distinguish these two causes of AKI using urine sodium, FENa, urine sediment, and reversibility.
• Primary vs Secondary Hyperparathyroidism: How to Tell Them ApartPrimary and secondary hyperparathyroidism both raise PTH, but they split on serum calcium. Learn to separate autonomous parathyroid disease from a compensatory response to CKD or vitamin D deficiency.
• Pulmonary Embolism vs Pneumonia: How to Tell Them ApartA high-yield USMLE comparison of pulmonary embolism and pneumonia, focusing on the key axis of vascular obstruction with often-quiet lungs versus parenchymal infection with consolidation, plus the tests that separate them.
• Rheumatoid Arthritis vs Osteoarthritis: How to Tell Them ApartRheumatoid arthritis versus osteoarthritis for the USMLE: distinguish autoimmune inflammatory synovitis from mechanical joint degeneration by joint distribution, morning stiffness, serology, and radiographic pattern.
• Sarcoidosis vs Idiopathic Pulmonary Fibrosis: How to Tell Them ApartSarcoidosis and idiopathic pulmonary fibrosis (IPF) are both restrictive interstitial lung diseases, but they differ on the key axis of granulomatous multisystem disease versus purely pulmonary fibrosis. Compare demographics, HRCT, histology, labs, and treatment.
• SIADH vs Diabetes Insipidus: How to Tell Them ApartSIADH and diabetes insipidus are opposite disorders of ADH. Learn to distinguish them using serum sodium, serum and urine osmolality, and the desmopressin challenge for USMLE success.
• Stable Angina vs Unstable Angina: How to Tell Them ApartDistinguish stable angina from unstable angina by pathophysiology, provoking factors, and pattern: a fixed supply-demand mismatch versus an acute plaque rupture event with thrombus formation.
• STEMI vs NSTEMI: How to Tell Them ApartSTEMI and NSTEMI both arise from acute plaque rupture and thrombosis, but they differ in the degree of coronary occlusion, ECG pattern, and reperfusion strategy. Learn to distinguish these two acute coronary syndromes for the USMLE.
• Systolic (HFrEF) vs Diastolic (HFpEF) Heart Failure: How to Tell Them ApartA high-yield USMLE comparison of systolic heart failure (HFrEF) versus diastolic heart failure (HFpEF), organized around ejection fraction, ventricular structure, exam findings, and treatment response.
• Transudative vs Exudative Pleural Effusion: How to Tell Them ApartCompare transudative and exudative pleural effusions for the USMLE: mechanism, causes, Light's criteria, and management—treat the systemic disease versus treat the local disease and drain.
• Type 1 vs Type 2 Diabetes: How to Tell Them ApartA high-yield comparison of Type 1 and Type 2 diabetes for the USMLE, centered on the core axis of autoimmune insulin deficiency versus insulin resistance, and how each shapes presentation, treatment, and complications.
• Vitamin B12 Deficiency vs Folate Deficiency: How to Tell Them ApartBoth cause megaloblastic macrocytic anemia, but only B12 deficiency causes neurologic damage. Learn to separate B12 from folate deficiency using MMA, neurologic findings, and body stores.