What is the most likely diagnosis for a 30-year-old man presenting with fever, shortness of breath, nonproductive cough, night sweats, and significant weight loss, with laboratory findings including leukocytosis (elevated white blood cell count), lymphopenia (low lymphocyte count), and elevated serum lactate dehydrogenase (LDH), and an arterial blood gas analysis showing hypoxemia (low partial pressure of oxygen) on room air?

Differential Diagnosis

• Single Most Likely Diagnosis
  • Pulmonary Tuberculosis (E): The patient's symptoms of fever, night sweats, weight loss, and a nonproductive cough for 6 weeks are classic for pulmonary tuberculosis. The presence of diffuse crackles over all lung fields and the chest x-ray findings (although not described, typically show upper lobe infiltrates or cavitations) also support this diagnosis.
• Other Likely Diagnoses
  • Pneumocystis jiroveci (formerly P. carini) pneumonia (D): Although the patient does not have a history of immunocompromise, Pneumocystis jiroveci pneumonia can occur in immunocompetent individuals, especially those with underlying lung disease. The patient's symptoms of fever, shortness of breath, and nonproductive cough, as well as the diffuse crackles on lung exam, are consistent with this diagnosis.
  • Viral Pneumonia (G): The patient's symptoms of fever, shortness of breath, and nonproductive cough, as well as the diffuse crackles on lung exam, could also be consistent with viral pneumonia, especially given the prolonged duration of symptoms.
• Do Not Miss Diagnoses
  • Lung Abscess (C): Although less likely given the patient's symptoms and lab results, lung abscess is a potentially life-threatening condition that requires prompt diagnosis and treatment. The presence of fever, weight loss, and night sweats could be consistent with a lung abscess, especially if the patient has a history of aspiration or underlying lung disease.
  • Streptococcal Pneumonia (F): Bacterial pneumonia, including streptococcal pneumonia, is a potentially life-threatening condition that requires prompt diagnosis and treatment. Although the patient's symptoms and lab results are not typical for bacterial pneumonia, it is still a possibility that should be considered.
• Rare Diagnoses
  • Acute Bronchitis (A): Acute bronchitis typically presents with a productive cough and is less likely to cause systemic symptoms such as fever, night sweats, and weight loss.
  • Bronchiectasis (B): Bronchiectasis is a chronic condition characterized by permanent dilation of the bronchi and is typically associated with a productive cough and recurrent infections. The patient's symptoms and lab results are not consistent with bronchiectasis.