Here are three interactive practice cases designed to sharpen your differential diagnosis skills. I recommend you grab a notebook and actively work through each one using the framework provided before checking the answers.

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🩺 Case 1: The Acute Abdomen

Patient: 42-year-old male, previously healthy
Chief Complaint: "My stomach hurts worse than anything I've ever felt."

History:

• Pain started 8 hours ago as vague periumbilical discomfort, now sharp and localized to RLQ
• Nausea x3 episodes, no vomiting yet
• No diarrhea, last BM yesterday
• No prior surgeries

Vitals: Temp 100.4°F (38°C), HR 110, BP 142/88, RR 18, SpO2 98% RA
Physical Exam:

• Awake, diaphoretic, moving uncomfortably on stretcher
• Abdomen: Guarding and rebound tenderness at McBurney's point, diminished bowel sounds
• Rectal: Normal tone, no masses, guaiac negative

Labs: WBC 14,200 with 85% neutrophils; Hgb 14.2; BMP normal; UA normal; lipase normal

Your Action Steps:

1. Generate your differential (list 5-7 possibilities ranked by lethality and probability)
2. Identify the "can't miss" diagnoses
3. What single test would绝大多数 narrow your differential?

<details> <summary>Click to reveal approach & answer</summary>

Prioritized Differential:

1. Acute Appendicitis (classic migration pattern, localized peritonitis)
2. Perforated Duodenal Ulcer (sudden onset, though pain usually epigastric/RUQ)
3. Cecal Diverticulitis (can mimic appendicitis, older patients usually)
4. Terminal Ileitis (Crohn's, Yersinia, or infectious)
5. Ureteral Stone (though pain usually flank→groin, UA would show hematuria)

Can't Miss: Appendiceal perforation with abscess, septic abdominal process

Next Step: CT abdomen/pelvis with IV contrast (or ultrasound if younger/alternative diagnosis needed)

</details>

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🫁 Case 2: The Subtle Dyspnea

Patient: 67-year-old female, retired teacher
Chief Complaint: "I just can't catch my breath like I used to."

History:

• Progressive exertional dyspnea over 6 weeks
• Dry cough, fatigue, 5 lb unintentional weight loss
• History: HTN, hypothyroidism (on levothyroxine), 30 pack-year smoking (quit 10 years ago)
• Meds: Lisinopril, levothyroxine, OTC ibuprofen PRN for back pain

Vitals: Temp 99.1°F, HR 94 irregularly irregular, BP 138/82, RR 20, SpO2 91% on room air
Physical Exam:

• Thin, elderly female, comfortable at rest
• Lungs: Fine bibasilar crackles, no wheezing
• Cardiac: Irregular rhythm, no murmurs, JVP 6 cm
• Extremities: No edema, no calf tenderness

Labs: Hgb 9.8 (MCV 82), Creatinine 1.4 (baseline 0.9), BNP 450, normal TSH
CXR: Bilateral lower lobe interstitial markings, small bilateral effusions

Your Action Steps:

1. Create a VINDICATE mnemonic differential (Vascular, Inflammatory, Neoplastic, Degenerative, Idiopathic, Congenital, Autoimmune, Traumatic, Endocrine)
2. Which finding is the "anchor" that changes everything?
3. What are your next 2 diagnostic steps?

<details> <summary>Click to reveal approach & answer</summary>

Key Anchor: New anemia (Hgb 9.8) + renal failure (Cr 1.4) + lung findings = Pulmonary-Renal Syndrome

Differential:

1. Goodpasture's Syndrome (Anti-GBM disease) - linear deposits
2. Granulomatosis with Polyangiitis (Wegener's) - c-ANCA positive
3. Microscopic Polyangiitis - p-ANCA positive
4. SLE (though less common at this age)
5. Rapidly Progressive Glomerulonephritis (Type III)

Why not CHF? BNP elevated but not sky-high; no peripheral edema; acute renal failure points to systemic vasculitis.

Next Steps:

1. Urinalysis with microscopy (look for dysmorphic RBCs, RBC casts)
2. Serologic testing: ANCA, anti-GBM antibodies, ANA, complements
3. Chest CT (to characterize infiltrates)
4. Nephrology consult (likely need renal biopsy)

</details>

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🌍 Case 3: The Febrile Traveler

Patient: 29-year-old graduate student
Chief Complaint: Fever and "weird spots" for 2 days

History:

• Returned 10 days ago from 3-week trip to rural Thailand and Cambodia
• Activities: Jungle trekking, temple visits, street food, unprotected fresh water swimming
• Symptoms: High fevers (up to 103°F), severe headache, retro-orbital pain, myalgias
• Rash started on wrists/ankles, spreading centrally (non-itchy)

Vitals: Temp 102.8°F, HR 118, BP 110/70, RR 22
Physical Exam:

• Ill-appearing, flushed
• Rash: Maculopapular on extremities, some petechiae on ankles
• Abdomen: Splenomegaly (2 cm below costal margin), tender
• Neurologic: Photophobia, neck supple

Labs: WBC 3,200 (low), Platelets 89,000 (low), Hgb 12.1, AST/ALT 120/85, Creatinine 1.2

Your Action Steps:

1. Geography-based differential: What diseases match Thailand/Cambodia + incubation period?
2. What finding on blood smear would confirm your diagnosis?
3. Intervention: Which empiric therapy should start NOW before labs return?

<details> <summary>Click to reveal approach & answer</summary>

Differential (Geographic):

1. Dengue Fever (classic: fever, thrombocytopenia, transaminitis, rash starting extremities - "breakbone fever")
2. Malaria (P. vivax or P. falciparum - though usually cyclical fevers, splenomegaly fits)
3. Scrub Typhus (rural Thailand, eschar may be missed in hairline/groin)
4. Leptospirosis (fresh water exposure, though usually conjunctival suffusion)
5. Typhoid Fever (rose spots, relative bradycardia often absent)
6. Chikungunya (similar to dengue but arthritis prominent)

Key Clues: Timing (10 days post-return), platelet count nadir, transaminitis, rash distribution

Blood Smear: Look for thrombocytopenia with hemoconcentration (HCT rise despite illness). For malaria: ring forms. For dengue: no specific findings but rules out malaria.

Immediate Action:

• Draw CBC, smear, type & screen (watch for bleeding)
• Start empiric doxycycline (covers scrub typhus, leptospirosis, rickettsial diseases)
• IV fluids cautiously (dengue caution: fluid overload risk when defervescence/plasma leak phase hits)
• Malaria rapid antigen test (stat)

</details>

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🎯 Your Clinical Reasoning Framework

For every case you encounter, run through this 5-Step Action Plan:

1. Anchor & Adjustment: Identify 2-3 objective findings that cannot be ignored (abnormal vitals, specific exam signs), then adjust for patient demographics
2. Rule Out the Reaper: List the top 3 life-threatening possibilities first (MI, PE, sepsis, intracranial bleed, etc.)
3. Pattern Recognition: Does this fit a classic presentation? If not, what's atypical?
4. Probabilistic Reasoning: What is prevalence in this population? (Bayesian thinking)
5. Test Threshold: What test result would change your management? Don't order if it won't change anything.

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To help me tailor more cases for you:

1. What's your training level? (Pre-clinical, clinical rotations, resident, or practicing clinician looking for CME?)
2. Which systems challenge you most? (Neuro, infectious disease, rheumatology, etc.)
3. Do you want cases focused on: Board exam prep, EM/acute care, outpatient ambulatory, or rare zebras?

I can generate 5 more cases in your target area with detailed teaching points on clinical reasoning bias (premature closure, anchoring bias, etc.).

Practicing Differential Diagnosis: Step-by-Step Guide

Differential diagnosis (DDx) is the systematic process of listing and narrowing down possible conditions based on a patient's history, exam, and tests. Practicing it builds clinical reasoning skills essential for medical students, residents, and clinicians. Here's how to take action and start practicing today.

Step 1: Learn the Core DDx Framework (5-10 Minutes to Review)

Before diving into cases:

1. Gather Data: Chief complaint, history of present illness (OPQRST: Onset, Provocation, Quality, Region/Radiation, Severity, Time course), PMH, meds, social hx, ROS.
2. Generate Broad DDx: Use VINDICATE mnemonic:
   • Vascular (e.g., MI, PE)
   • Infectious/Inflammatory (e.g., pneumonia, appendicitis)
   • Neoplastic (e.g., cancer)
   • Degenerative/Drugs (e.g., dementia, toxicity)
   • Iatrogenic/Immune (e.g., allergy)
   • Congenital (e.g., defects)
   • Autoimmune (e.g., lupus)
   • Trauma/Toxins (e.g., fracture, overdose)
   • Endocrine/Electrolytes/Metabolic (e.g., DKA, hyponatremia)
3. Prioritize: Rank by likelihood (common > zebras), severity (life-threatening first), treatability.
4. Refine: Order targeted tests (e.g., labs, imaging); reassess.
5. Document: Write your top 3-5 differentials with rationale.

Pro Tip: Time yourself—aim for 2-5 minutes per initial DDx.

Step 2: Use Free Interactive Resources (Start Here for Hands-On Practice)

These sites offer real clinical cases with histories, exams, images, and feedback. No login required for most starters.

• Geeky Medics OSCE Cases (Best for Beginners/UK MLA Prep): 50+ interactive scenarios. Read presentation, take history/exam, build DDx, reveal diagnosis.

  • Start Here
  • Examples:

    Case	Presentation	Why Practice DDx?
    Painful Hand	54yo woman with painful hand	Rheumatology vs infection vs trauma
    Dizziness & Nausea	42yo woman	Vertigo vs cardiac vs neuro
    Paediatric Rash	6yo boy with acute rash	Viral vs bacterial vs allergic
    Cough, Chest Pain, Fever	34yo man	Pneumonia vs PE vs MI

• LITFL Clinical Cases (250+ Q&A, Great for ED/Emergency): Image-heavy (ECGs, X-rays). Read case, guess Dx, check discussion.

  • Start Here
  • Examples: "ROSC, hypotension and ECG" (post-arrest DDx), "Wide Complex Tachycardia" (arrhythmia differentials).

• Human Diagnosis Project (Human Dx): Free interactive app/site. Solve peer-reviewed cases (e.g., clozapine myocarditis, pyoderma gangrenosum). Update your DDx list; get scored.

  • Join & Practice – Sign up for Global Morning Report.

• Other Top Free Tools:

  Resource	Focus	Link
  Diagnosaurus (AccessMedicine)	Symptom-based DDx lists	accessmedicine.mhmedical.com/Diagnosaurus.aspx
  Oxford Medical Education	DDx by condition (e.g., motor neuron disease)	oxfordmedicaleducation.com/category/differential-diagnosis
  100 Cases in Clinical Medicine (PDF)	Download & solve offline	Free PDF

Step 3: Daily Practice Routine (Get Started in 15-30 Minutes/Day)

1. Pick 1-3 cases from above (e.g., Geeky Medics).
2. No peeking: Write your DDx on paper/app (top 5, ranked).
3. Review history/exam step-by-step.
4. Refine DDx after "tests."
5. Compare to answer; note misses (e.g., "Forgot PE in chest pain").
6. Track in a journal: 1 case/day → mastery in weeks.
7. Level up: Discuss on Reddit (r/medicine, r/medicalschool) or podcasts like "The Clinical Problem Solvers."

Quick Sample Case to Try Now (Abdominal Pain – Common Starter)

Patient: 32yo male, sudden right lower quadrant pain (8/10), nausea/vomiting, fever 38.5°C. No trauma, appendectomy at 15yo. Exam: Rebound tenderness RLQ.

• Your Turn: List top 3 DDx + next steps.
• Hints: Use VINDICATE. (Answer: Appendicitis #1, but consider ectopic/Diverticulitis/OVT.)

Practice More: Head to Geeky Medics for full interactive versions.

What chief complaint or specialty (e.g., cardiology, peds, neuro) do you want cases for? Beginner or advanced level? Share a sample DDx you're working on for feedback!