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 65-year-old man presents with low-grade fever and malaise for the last 4 months. He also says he has lost 9 kg (20 lb) during this period and suffers from extreme fatigue. Past medical history is significant for a mitral valve replacement 5 years ago. His temperature is 38.1°C (100.6°F), respirations are 22/min, pulse is 102/min, and blood pressure is 138/78 mm Hg. On physical examination, there is a new onset 2/6 holosystolic murmur loudest in the apical area of the precordium. Which of the following organisms is the most likely cause of this patient’s condition?

Coagulase-negative Staphylococcus spp.

A 9-year-old boy with cystic fibrosis (CF) presents to the clinic with fever, increased sputum production, and cough. The vital signs include: temperature 38.0°C (100.4°F), blood pressure 126/74 mm Hg, heart rate 103/min, and respiratory rate 22/min. His physical examination is significant for short stature, thin body frame, decreased breath sounds bilateral, and a 2/6 holosystolic murmur heard best on the upper right sternal border. His pulmonary function tests are at his baseline, and his sputum cultures reveal Pseudomonas aeruginosa. What is the best treatment option for this patient?

Dornase alfa 2.5 mg as a single-use

Sulfamethoxazole and trimethoprim for 14 days

A 13-year-old boy is brought by his mother to the emergency department because he has had fever, chills, and severe coughing for the last two days. While they originally tried to manage his condition at home, he has become increasingly fatigued and hard to arouse. He has a history of recurrent lung infections and occasionally has multiple foul smelling stools. On presentation, his temperature is 102.2 °F (39 °C), blood pressure is 106/71 mmHg, pulse is 112/min, and respirations are 20/min. Physical exam reveals scattered rhonchi over both lung fields, rales at the base of the right lung base and corresponding dullness to percussion. The most likely organism responsible for this patient's symptoms has which of the following characteristics?

Coagulase-positive, gram-positive cocci

A 45-year-old male presents to his primary care physician complaining of drainage from his left great toe. He has had an ulcer on his left great toe for over eight months. He noticed increasing drainage from the ulcer over the past week. His past medical history is notable for diabetes mellitus on insulin complicated by peripheral neuropathy and retinopathy. His most recent hemoglobin A1c was 9.4%. He has a 25 pack-year smoking history. He has multiple sexual partners and does not use condoms. His temperature is 100.8°F (38.2°C), blood pressure is 150/70 mmHg, pulse is 100/min, and respirations are 18/min. Physical examination reveals a 1 cm ulcer on the plantar aspect of the left great toe surrounded by an edematous and erythematous ring. Exposed bone can be palpated with a probe. There are multiple small cuts and bruises on both feet. A bone biopsy reveals abundant gram-negative rods that do not ferment lactose. The pathogen most likely responsible for this patient’s current condition is also strongly associated with which of the following conditions?

Waterhouse-Friedrichsen syndrome

An 8-year-old boy is brought to the emergency department because of a 4-day history of severe, left-sided ear pain and purulent discharge from his left ear. One week ago, he returned with his family from their annual summer vacation at a lakeside cabin, where he spent most of the time outdoors hiking and swimming. Examination shows tragal tenderness and a markedly edematous and erythematous external auditory canal. Audiometry shows conductive hearing loss of the left ear. Which of the following is the most likely cause of this patient's symptoms?

Infection with Pseudomonas aeruginosa

Pleomorphic replacement of normal bone

Abnormal epithelial growth on tympanic membrane

Infection with varicella zoster virus

Infection with Aspergillus species

Pseudomonas aeruginosa for USMLE Step 2 CK: High-Yield Microbiology Lessons

Microbiology - The Blue-Green Menace

Pseudomonas aeruginosa on cetrimide agar with pyocyanin

Key Features: Gram-negative rod, motile, strictly aerobic.
Metabolism: Oxidase-positive, non-lactose fermenting.
Hallmarks: Produces a blue-green pigment (pyocyanin) and has a characteristic grape-like odor.
Culture: Grows on MacConkey agar.

⭐ It is a notorious opportunistic pathogen, especially in patients with cystic fibrosis, burns, and those who are immunocompromised. It is a common cause of ventilator-associated pneumonia.

📌 PSEUDOMONAS mnemonic: Pneumonia, Sepsis, External otitis, UTIs, Drug use, Osteomyelitis, Meningitis, And Skin infections (hot tub folliculitis).

Virulence Factors - Arsenal of Attack

Virulence Factor	Mechanism of Action
Exotoxin A	Inactivates Elongation Factor 2 (EF-2) via ADP-ribosylation, halting protein synthesis.
Phospholipase C	Degrades cellular membranes, contributing to tissue damage and hemolysis.
Pyocyanin	Blue-green pigment; generates reactive oxygen species (ROS), impairing ciliary function.
Endotoxin (LPS)	Outer membrane component; triggers robust inflammatory response leading to fever & shock.
Biofilm	Polysaccharide slime that protects from antibiotics & phagocytosis; crucial in CF & device infections.

⭐ Exam Favorite: The mechanism of Exotoxin A (ADP-ribosylation of EF-2) is identical to that of Diphtheria toxin.

Clinical Diseases - The Opportunist's Hitlist

P. aeruginosa is a quintessential opportunist, primarily striking individuals with compromised defenses. It's notorious for causing a wide array of infections, especially in hospitalized patients.

📌 Mnemonic: BE PSEUDO

Burns: A leading cause of infection in burn victims, thriving in the moist, avascular environment of the eschar.
Endocarditis: Tricuspid valve endocarditis, particularly in IV drug users.
Pneumonia: The classic pulmonary pathogen in cystic fibrosis patients, leading to bronchiectasis. Also, a major cause of hospital-acquired and ventilator-associated pneumonia (VAP).
Sepsis: Can cause life-threatening sepsis, often originating from lung or urinary tract infections, especially in neutropenic patients.
Ecthyma Gangrenosum: Hemorrhagic, necrotic skin lesions pathognomonic for Pseudomonas sepsis.
UTIs: Complicated urinary tract infections, frequently associated with indwelling catheters.
Diabetic Osteomyelitis: Chronic bone infection, classically following puncture wounds through footwear in diabetics.
Otitis: Malignant otitis externa in the elderly and diabetics; "swimmer's ear" and hot tub folliculitis.

Ecthyma gangrenosum from Pseudomonas aeruginosa

⭐ P. aeruginosa is the most common cause of ventilator-associated pneumonia (VAP), a critical consideration for patients on mechanical ventilation in the ICU.

Treatment - The Resistance Fight

P. aeruginosa is notorious for its multi-drug resistance (MDR), often requiring combination therapy guided by local sensitivity data.

Drug Class	Key Anti-Pseudomonals
Penicillins	Piperacillin-tazobactam
Cephalosporins	Ceftazidime, Cefepime
Carbapenems	Meropenem, Imipenem
Monobactams	Aztreonam
Fluoroquinolones	Ciprofloxacin
Aminoglycosides	Gentamicin, Tobramycin

A ubiquitous Gram-negative rod, it is oxidase-positive and thrives in aquatic environments.

It is a classic opportunistic pathogen, causing severe disease in patients with cystic fibrosis, burns, and neutropenia.

Key clinical syndromes include hot tub folliculitis, malignant otitis externa, and ventilator-associated pneumonia.

It produces a blue-green pigment (pyocyanin) and a characteristic grape-like odor.

Its major virulence factor is Exotoxin A, which inhibits protein synthesis by inactivating EF-2.

Notoriously multidrug-resistant.

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