Differential Diagnosis for a 55-year-old Male with Respiratory and Systemic Symptoms

Single Most Likely Diagnosis

• Community-Acquired Pneumonia (CAP): The patient's symptoms of cough with expectoration, body pain, fever, elevated CRP, and neutrophilia are highly suggestive of a bacterial infection, with CAP being a leading consideration given the clinical presentation and laboratory findings.

Other Likely Diagnoses

• Influenza: Although the patient's symptoms could also align with influenza, especially during flu season, the presence of neutrophilia and elevated CRP levels leans more towards a bacterial infection. However, influenza can sometimes present with similar laboratory findings, especially if there's a secondary bacterial infection.
• Chronic Bronchitis Exacerbation: If the patient has a history of chronic bronchitis, an exacerbation could present with similar symptoms. The elevated neutrophil count and CRP support an infectious or inflammatory cause.
• Acute Bronchitis: This is less likely given the duration of symptoms and the presence of fever and body pain, which are more suggestive of a lower respiratory tract infection like pneumonia.

Do Not Miss Diagnoses

• Tuberculosis (TB): Although less likely given the acute presentation, TB can present with similar symptoms and must be considered, especially in regions with high TB prevalence. A missed diagnosis of TB can have significant consequences.
• Lung Abscess: This condition can present with fever, cough, and body pain, and while less common, it's a serious condition that requires prompt diagnosis and treatment.
• Sepsis: If the patient's condition worsens, sepsis should be considered, especially with signs of systemic inflammation (elevated CRP, neutrophilia). Early recognition and treatment of sepsis are critical.

Rare Diagnoses

• Q Fever: Caused by Coxiella burnetii, this infection can present with similar symptoms and is more likely in individuals with exposure to livestock. It's a rare cause of pneumonia but should be considered in the appropriate epidemiological context.
• Psittacosis: This is a rare infectious disease caused by Chlamydophila psittaci, typically associated with exposure to infected birds. It can cause pneumonia and should be considered in patients with a relevant exposure history.

Given the information provided, the patient's presentation is most consistent with a bacterial infection, likely community-acquired pneumonia, based on the symptoms and laboratory findings. However, it's essential to consider other diagnoses and to tailor the diagnostic workup and treatment based on the patient's specific risk factors, exposure history, and clinical course.