A 7-year-old girl is brought to the pediatrician by her parents for red papules over her left thigh and swelling in the right axilla for the past few days. Her parents say that she had a cat bite on her left thigh 2 weeks ago. Her temperature is 38.6°C (101.4°F), pulse is 90/min, and respirations are 22/min. On her physical examination, hepatosplenomegaly is present with a healing area of erythema on her left thigh. Her laboratory studies show: Hemoglobin 12.9 gm/dL Leukocyte count 9,300/mm3 Platelet count 167,000/mm3 ESR 12 mm/hr Which of the following is the most appropriate next step in management?

Surgical excision of the lymph node

A 62-year-old woman presents to the emergency department for evaluation of a spreading skin infection that began from an ulcer on her foot. The patient has type 2 diabetes mellitus that is poorly controlled. On examination, there is redness and erythema to the lower limb with skin breakdown along an extensive portion of the leg. The patient’s tissues separate readily from the fascial plane, prompting a diagnosis of necrotizing fasciitis. What is the exotoxin most likely associated with this patient’s presentation?

Streptococcal pyogenic exotoxin B

Streptococcal pyogenic exotoxin A

A 35-year-old man comes to the emergency room for severe left leg pain several hours after injuring himself on a gardening tool. His temperature is 39°C (102.2°F) and his pulse is 105/min. Physical examination of the left leg shows a small laceration on the ankle surrounded by dusky skin and overlying bullae extending to the posterior thigh. There is a crackling sound when the skin is palpated. Surgical exploration shows necrosis of the gastrocnemius muscles and surrounding tissues. Tissue culture shows anaerobic gram-positive rods and a double zone of hemolysis on blood agar. Which of the following best describes the mechanism of cellular damage caused by the responsible pathogen?

Degradation of cell membranes by phospholipase

Inhibition of neurotransmitter release by protease

Inactivation of elongation factor by ribosyltransferase

Lipopolysaccharide-induced complement and macrophage activation

Increase of intracellular cAMP by adenylate cyclase

A 24-year-old man is brought to the emergency department after he is found sluggish, drowsy, feverish, and complaining about a headache. His past medical history is unremarkable. His vital signs include: blood pressure 120/60 mm Hg, heart rate 70/min, respiratory rate 17/min, and body temperature 39.0°C (102.2°F). On physical examination, the patient is dysphasic and incapable of following commands. Gait ataxia is present. No meningeal signs or photophobia are present. A noncontrast CT of the head is unremarkable. A T2 MRI is performed and is shown in the image. A lumbar puncture (LP) is subsequently performed. Which of the following CSF findings would you most likely expect to find in this patient?

Opening pressure: 15 cm H2O, color: clear, protein: 50 mg/dL, cell count: 40 cells/µL, mostly lymphocytes, ratio CSF:blood glucose: 0.65

Opening pressure: 28 cm H2O, color: cloudy, protein: 68 mg/dL, cell count: 150 cells/µL, mostly PMNs, ratio CSF:blood glucose: 0.3

Opening pressure: 38 cm H2O, color: cloudy, protein: 75 mg/dL, cell count: 80 cells/µL, mostly lymphocytes, ratio CSF:blood glucose: 0.25

Opening pressure: 18 cm H2O, color: clear, protein: 40 mg/dL, cell count: 2 cells/µL, mostly polymorphonuclear (PMNs), ratio CSF:blood glucose: 0.7

Opening pressure: 40 cm H2O, color: cloudy, protein: 80 mg/dL, cell count: 135 cells/µL, mostly lymphocytes with some PMNs, ratio CSF:blood glucose: 0.2

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

A 43-year-old man is brought to the emergency department 40 minutes after falling off a 10-foot ladder. He has severe pain and swelling of his right ankle and is unable to walk. He did not lose consciousness after the fall. He has no nausea. He appears uncomfortable. His temperature is 37°C (98.6°F), pulse is 98/min, respirations are 16/min, and blood pressure is 110/80 mm Hg. He is alert and oriented to person, place, and time. Examination shows multiple abrasions over both lower extremities. There is swelling and tenderness of the right ankle; range of motion is limited by pain. The remainder of the examination shows no abnormalities. An x-ray of the ankle shows an extra-articular calcaneal fracture. Intravenous analgesia is administered. Which of the following is the most appropriate next step in the management of this patient?

Short leg splint and orthopedic consultation

Broad-spectrum antibiotic therapy

Open reduction and internal fixation

Skin and soft tissue infections for USMLE Step 3: High-Yield Internal Medicine Lessons

SSTI Basics - The Layer Cake

Skin infection depth and etiology

Concept: Infections classified by the anatomical layer they occupy, from superficial to deep. Deeper infections are more severe.

⭐ Necrotizing Fasciitis: A surgical emergency. Suspect when a patient presents with signs of cellulitis but has excruciating pain disproportionate to the physical exam findings. Crepitus from subcutaneous gas is a late, ominous sign.

Necrotizing Infections - A True Emergency

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Treatment Strategy - The Right Tools

Core Principle: Differentiate purulent vs. non-purulent infections to guide therapy.
Incision & Drainage (I&D): Cornerstone for purulent infections (abscesses, furuncles).
- Send wound cultures if available, especially in treatment failure or severe cases.

Severe / Necrotizing Fasciitis: Surgical emergency!
- Requires immediate surgical debridement.
- Broad-spectrum antibiotics: Vancomycin/Linezolid + Piperacillin-Tazobactam OR a Carbapenem.

⭐ Exam Favorite: A patient history of a "spider bite" that worsens is a classic clinical vignette for community-acquired MRSA (CA-MRSA).

High-Yield Points - ⚡ Biggest Takeaways

Erysipelas features sharply-demarcated, raised erythema, almost always from Strep. pyogenes.

Cellulitis has indistinct borders and involves deeper dermis, caused by S. aureus or Strep. pyogenes.

Necrotizing fasciitis is a surgical emergency signaled by pain out of proportion to exam and crepitus.

An abscess requires incision and drainage; always consider MRSA as a likely pathogen.

Diabetic foot infections are typically polymicrobial, requiring broad-spectrum antibiotics and debridement.

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