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Difficulty: HardInfectious Diseases

Following a deceased-donor renal allograft transplantation eight months ago, a 58-year-old woman is admitted to the hospital due to a three-week history of productive cough, night sweats, and new-onset left-sided hemiparesis. Her immunosuppressive regimen includes tacrolimus, mycophenolate mofetil, and prednisone. Routine post-transplant antimicrobial prophylaxis was discontinued two months ago. Vital signs show a temperature of 38.2°C (100.8°F). Neurological examination reveals 3/5 strength in the left upper and lower extremities. A chest radiograph demonstrates a large cavitary lesion in the right upper lobe. Subsequent magnetic resonance imaging of the brain reveals a 2.5-cm ring-enhancing lesion in the right frontal lobe with surrounding vasogenic edema. Bronchoscopy with bronchoalveolar lavage is performed, and cytological analysis reveals branching, filamentous, Gram-positive rods that stain weakly positive with a modified acid-fast stain. Which of the following is the most appropriate initial targeted pharmacotherapy for this patient's underlying infection?

  1. Trimethoprim-sulfamethoxazoleAnswer
  2. B
    Penicillin G
  3. C
    Voriconazole
  4. D
    Isoniazid, rifampin, pyrazinamide, and ethambutol
  5. E
    Pyrimethamine and sulfadiazine

Answer

Trimethoprim-sulfamethoxazole
The correct answer is trimethoprim-sulfamethoxazole. This immunocompromised patient's presentation of a cavitary lung lesion and a ring-enhancing brain abscess is highly characteristic of disseminated nocardiosis. The diagnosis is definitively supported by the bronchoalveolar lavage findings: branching, filamentous, Gram-positive rods that are weakly positive on modified acid-fast staining. Nocardia species are aerobic, soil-dwelling bacteria that cause opportunistic pulmonary infections with a strong propensity for hematogenous dissemination to the central nervous system. Trimethoprim-sulfamethoxazole (TMP-SMX) is the cornerstone of targeted therapy for nocardiosis. In severe or disseminated cases, a second agent like a carbapenem (e.g., imipenem) or amikacin may be added empirically, but TMP-SMX remains the essential targeted agent.

Step-by-Step Solution

1
Analyze the patient's immune status and clinical presentation.
The patient is an immunocompromised solid organ transplant recipient presenting with a dual-organ syndrome: a cavitary pulmonary infection and a focal central nervous system (CNS) abscess.
Identifying the pattern of organ involvement narrows the differential diagnosis of opportunistic infections to pathogens that typically disseminate from the lungs to the brain (e.g., Nocardia, Aspergillus, Tuberculosis).
2
Interpret the specific microbiological findings from the bronchoalveolar lavage.
The pathogen is described as branching, filamentous, Gram-positive rods that are weakly acid-fast.
These specific staining characteristics are pathognomonic for Nocardia species. They differentiate the organism from Actinomyces (which is not acid-fast) and Aspergillus (which is a fungus, not a Gram-positive bacterium).
3
Determine the appropriate targeted pharmacotherapy for the identified pathogen.
Trimethoprim-sulfamethoxazole is selected as the cornerstone of therapy.
Trimethoprim-sulfamethoxazole is the standard first-line targeted therapy for both pulmonary and disseminated (CNS) nocardiosis.

Key Concept

Nocardiosis presents as cavitary lung lesions and brain abscesses in immunocompromised patients, diagnosed by weakly acid-fast, branching Gram-positive rods, and is treated with trimethoprim-sulfamethoxazole.
Estimated Time:1m 30s
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Topics

Infectious Diseases

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