# Clinical Cases: Advanced Diagnostic Reasoning

## Case 1: Anchoring Bias and Premature Closure

### Clinical Image
![Chest X-ray demonstrating lung pathology requiring careful diagnostic reasoning](case_01_image.jpg)
*Source: [Wikimedia Commons - Chest X-ray](https://commons.wikimedia.org/wiki/Category:Chest_radiographs) - CC BY-SA 4.0*

### Case Presentation
A 58-year-old woman with history of recurrent urinary tract infections presents to the emergency department with fever, dysuria, and suprapubic discomfort. Urinalysis shows pyuria with positive leukocyte esterase and nitrites. She is diagnosed with UTI and started on ciprofloxacin. She is admitted for IV antibiotics given mild acute kidney injury (creatinine 1.6 mg/dL from baseline 1.0 mg/dL). Despite 48 hours of appropriate antibiotics, her fever persists at 38.9 degrees Celsius, and she develops new confusion. The admitting team continues the UTI treatment plan. The covering sub-intern, reviewing the case during sign-out, asks "what else could this be?" and notes that the patient's confusion seems disproportionate to her degree of illness. Physical examination reveals subtle nuchal rigidity that was not previously documented. The sub-intern raises this concern with the senior resident, who agrees to perform a lumbar puncture. CSF analysis reveals 1,200 WBCs with 90% neutrophils, protein 180 mg/dL, and glucose 25 mg/dL (serum glucose 120 mg/dL). The patient has bacterial meningitis, likely from hematogenous spread. Broad-spectrum meningitis coverage is initiated, and she ultimately recovers with IV ceftriaxone and vancomycin for 14 days.

### Key Learning Points
- Anchoring bias occurs when clinicians fixate on the initial diagnosis (UTI) and fail to adequately adjust when new information emerges (persistent fever, confusion)
- Premature closure means stopping the diagnostic search after finding one diagnosis without asking "what else could this be?"
- The "worst case scenario" or "can't miss" diagnoses should be considered for every presentation; meningitis should be on the differential for fever with altered mental status
- Speaking up about diagnostic concerns, regardless of hierarchy, can prevent missed diagnoses and improve patient outcomes

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## Case 2: Bayesian Reasoning and Test Interpretation

### Clinical Image
![D-dimer test result laboratory slip](case_02_image.jpg)
*Source: [Wikimedia Commons - Laboratory Medicine](https://commons.wikimedia.org/wiki/Category:Clinical_laboratory) - Public Domain*

### Case Presentation
A 25-year-old healthy woman presents with 2 days of left-sided pleuritic chest pain after a 4-hour car trip. She has no leg swelling, no prior VTE, no recent surgery, and is not on estrogen-containing contraceptives. Vital signs are normal with oxygen saturation of 99% on room air. Her Wells score for PE is 0 (low probability). The emergency physician orders a D-dimer, which returns elevated at 620 ng/mL (normal less than 500 ng/mL). The EP plans to order a CT pulmonary angiography. The medicine sub-intern asks about the pre-test probability. The supervising resident explains that with a Wells score of 0 and normal vital signs, her pre-test probability is approximately 3-5%. A D-dimer has sensitivity of approximately 95% and specificity of approximately 40% in this population. Using the Fagan nomogram or likelihood ratio calculation, even with a positive D-dimer (LR+ approximately 1.6), her post-test probability only rises to approximately 5-8%. The team considers whether the low positive D-dimer could be a false positive. Upon further history, the patient mentions she has had a cold for the past week with mild productive cough. Inflammation from viral infection commonly elevates D-dimer. After shared decision-making about the low probability of PE versus radiation exposure from CT, the patient opts for close follow-up rather than CT-PA. Her symptoms resolve over 5 days without treatment.

### Key Learning Points
- Pre-test probability should be estimated before ordering tests; when pre-test probability is very low, even positive results may represent false positives
- Likelihood ratios quantify how much a test result changes probability: LR+ greater than 10 strongly increases probability, while LR+ of 1-2 provides minimal diagnostic value
- D-dimer has high sensitivity but low specificity, making it useful for ruling out PE in low-probability patients but not for ruling in PE
- Clinical decision rules (Wells, PERC) should guide test ordering; ordering tests indiscriminately leads to false positive cascades

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## Case 3: Diagnostic Uncertainty and Watchful Waiting

### Clinical Image
![Physician reviewing diagnostic imaging with patient](case_03_image.jpg)
*Source: [Wikimedia Commons - Medical Imaging](https://commons.wikimedia.org/wiki/Category:Medical_imaging) - CC BY-SA 3.0*

### Case Presentation
A 45-year-old man presents with 3 weeks of intermittent low-grade fever (maximum 38.3 degrees Celsius), fatigue, and 5-pound weight loss. He has no localizing symptoms: no cough, no urinary symptoms, no diarrhea, no joint pain, no rash. Physical examination is unremarkable except for possible splenomegaly. Initial workup shows mild normocytic anemia (hemoglobin 11.2 g/dL), elevated ESR (65 mm/hr), elevated LDH (280 U/L), and mildly elevated alkaline phosphatase. Chest X-ray is normal. CT of chest, abdomen, and pelvis shows borderline splenomegaly (14 cm) but no lymphadenopathy or masses. Blood cultures are negative. The team considers the broad differential for fever of unknown origin: infection (endocarditis, occult abscess, tuberculosis), malignancy (lymphoma, leukemia), and autoimmune disease (Still's disease, vasculitis). Echocardiography shows no vegetations. ANA and RF are negative. Peripheral smear is unremarkable. The attending suggests a "diagnostic time-out" to reformulate the problem. Rather than ordering more tests immediately, the team decides on watchful waiting with close follow-up in 1 week. At follow-up, the patient reports night sweats and notices a new enlarged lymph node in his left axilla. Repeat examination confirms 3 cm rubbery lymphadenopathy. Excisional biopsy reveals Hodgkin lymphoma. The 1-week delay allowed the disease to declare itself, avoiding unnecessary invasive testing while the diagnosis was evolving.

### Key Learning Points
- Time can be a diagnostic tool; watchful waiting with close follow-up allows evolving presentations to declare themselves
- Fever of unknown origin (greater than 38.3 degrees Celsius for greater than 3 weeks without diagnosis) has classic categories: infection, malignancy, and autoimmune/inflammatory conditions
- A "diagnostic time-out" forces reconsideration of the working diagnosis and prevents reflexive test ordering
- When the diagnosis is unclear, focus on the "most likely," "most dangerous," and "most treatable" conditions while monitoring for new clinical clues