USMLE Clinical Cases: Fat Embolism, Ulcerative Colitis and Intussusception

These 3 detailed USMLE clinical case questions walk through the reasoning step by step: fat embolism syndrome after a femur fracture, how smoking helps separate ulcerative colitis from Crohn disease, and the management of intussusception in an infant. Each case explains why the correct answer is right and why the other choices are wrong.

Written by Dr. Ray Makar, MD, author of the Repeated USMLE Questions eBook series.

Prefer video? Watch daily USMLE case questions on MedFocus by Dr. Ray on YouTube.

How to use this page: read the question, pick your answer, then tap Show Answer and Explanation.

Questions

Question 1. A 55-year-old man underwent internal fixation of a fractured femur 24 hours ago after a motor vehicle accident. He was stable after the surgery. Now he complains of sudden shortness of breath and he is confused and drowsy. His temperature is 100.6 F, pulse is 118/min, respiratory rate is 28/min, blood pressure is 128/78 mmHg and oxygen saturation is 86% on room air. Examination reveals petechiae over his chest, axillae and conjunctivae, and scattered crackles at both lung bases. Chest x-ray shows bilateral diffuse infiltrates. ECG shows sinus tachycardia. Cardiac enzymes are normal. Which of the following is the most likely diagnosis?

A- Acute myocardial infarction
B- Cardiac tamponade
C- Fat embolism syndrome
D- Pulmonary thromboembolism
E- Septic shock

C- Fat embolism syndrome- This patient has the classic triad of fat embolism syndrome: respiratory distress (hypoxemia, bilateral infiltrates), neurologic changes (confusion, drowsiness) and a petechial rash on the upper body, axillae and conjunctivae. It typically appears 24 to 72 hours after a fracture of a long bone such as the femur, or after orthopedic surgery, when fat globules from the bone marrow enter the circulation and lodge in the lungs, brain and skin.
Treatment is supportive: oxygen and mechanical ventilation if needed, with fluids to keep him stable. Early fixation of long-bone fractures lowers the risk.
Why the other choices are wrong:
A- Acute myocardial infarction: normal cardiac enzymes and no ischemic ECG changes; it does not explain the petechiae.
B- Cardiac tamponade: causes hypotension, distended neck veins and muffled heart sounds (Beck triad); this patient’s blood pressure is normal.
D- Pulmonary thromboembolism: also causes sudden dyspnea and hypoxemia after orthopedic surgery, but not confusion with a petechial rash, and the chest x-ray is usually near normal.
E- Septic shock: he is not hypotensive and there is no source of infection.
The correct answer is C

 

Question 2. A 27-year-old man complains of a 2-week history of watery diarrhea and 5 lb. weight loss. He had no similar episodes in the past, did not travel recently, and did not eat out or take antibiotics. He quit smoking last month after 1 PPD for 5 years. His family history is significant of some relatives who are diagnosed with inflammatory bowel disease. His examination reveals diffuse mild abdominal tenderness and hyperactive bowel sounds. Stool analysis is negative for blood and ova. Which of the following supports the diagnosis of ulcerative colitis rather than Crohn’s disease in this patient?

A- History of diarrhea and weight loss
B- Family history of inflammatory bowel disease
C- Smoking cessation
D- Negative stool analysis
E- Having the first attack in his twenties

Smoking has opposite effects on the two main inflammatory bowel diseases:
Ulcerative colitis: Smoking is protective. Stopping smoking can trigger or worsen UC.
Crohn’s disease: Smoking worsens disease activity.
This patient quit smoking recently, which supports ulcerative colitis rather than Crohn’s disease.
Why the other options don’t distinguish UC from Crohn’s:
A — Diarrhea and weight loss: Seen in both UC and Crohn’s.
B — Family history of inflammatory bowel disease: Increases risk of both.
D — Negative stool analysis: Helps exclude infection, but does not differentiate UC vs Crohn’s.
E — First attack in his twenties: Typical age of onset for both conditions.
The correct answer is C
Key takeaway:👉 Smoking cessation favors ulcerative colitis, while active smoking is more associated with Crohn’s disease.

 

Question 3. An 8-month-old boy presents to the clinic with his mother. She tells you that her son had several bowel movements today, and his stool was like red jelly. He vomited all the food and water. He is also weak, fussy, and holding his belly. He has no fever or signs of dehydration. On abdominal examination, there is a palpable abdominal mass in the right hypochondrium and emptiness in the right lower abdomen. Ultrasonography is performed to confirm the diagnosis. What is the next step of management for this patient?

A- MRI of the abdomen
B- Colonoscopy
C- IV hydration and antibiotics
D- Contrast enema
E- Refer to surgery for bowel resection

The presentation is classic for intussusception:
“Red jelly” stool (currant jelly)
Intermittent abdominal pain, vomiting
Palpable abdominal mass (sausage-shaped)
Empty right lower quadrant (Dance sign)
Ultrasound confirms the diagnosis.
The next step in management (if no perforation, shock, or peritonitis) is a contrast enema (air or barium), which is both diagnostic and therapeutic by reducing the intussusception.
Why the other options are wrong:
A — MRI: Not needed; ultrasound is sufficient.
B — Colonoscopy: No role in acute pediatric intussusception.
C — IV hydration and antibiotics: Supportive only; does not treat the cause.
E — Surgery: Reserved for failed enema reduction or complications (perforation, peritonitis).
The correct answer is D
Key takeaway:👉 Stable infant with confirmed intussusception → contrast (air) enema is first-line treatment.

 

High-yield points from this set

  • Hypoxemia + confusion + petechiae 24–72 hours after a long-bone fracture = fat embolism syndrome.
  • Stopping smoking can trigger ulcerative colitis; smoking worsens Crohn disease.
  • Currant-jelly stool, colicky pain and a sausage-shaped mass in an infant = intussusception; air or contrast enema both confirms and treats it.

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