# Clinical Cases: Pulmonary Infections

## Case 1: Community-Acquired Pneumonia (Streptococcus pneumoniae)

### Clinical Image
![Chest X-ray showing lobar pneumonia](case_01_image.jpg)
*Source: [Wikimedia Commons - Lobar Pneumonia X-ray](https://commons.wikimedia.org/wiki/File:X-ray_of_lobar_pneumonia.jpg) - CC BY-SA 4.0*

### Case Presentation
A 58-year-old man with a history of hypertension and diabetes presents with 3 days of fever, productive cough with rust-colored sputum, and right-sided pleuritic chest pain. He reports sudden onset of shaking chills on the first day of illness. He has no recent travel or sick contacts. On examination, he appears ill and is febrile to 39.4C. Vital signs show heart rate 105 bpm, blood pressure 128/78 mmHg, respiratory rate 24/min, and oxygen saturation 92% on room air.

Chest examination reveals dullness to percussion, increased tactile fremitus, bronchial breath sounds, and egophony ("E to A" change) over the right lower lobe. These findings indicate lobar consolidation.

CURB-65 score: Confusion (no) = 0, Urea greater than 7 mmol/L (BUN 24 mg/dL, yes) = 1, Respiratory rate greater than 30 (no) = 0, Blood pressure less than 90 systolic (no) = 0, Age 65 or older (no) = 0. Total score = 1, suggesting outpatient treatment may be appropriate, but given his comorbidities and hypoxemia, hospital admission is warranted.

Chest X-ray shows dense consolidation of the right lower lobe with air bronchograms, the classic appearance of lobar pneumonia. Sputum Gram stain reveals numerous neutrophils and gram-positive diplococci. Blood cultures and sputum culture are obtained. Procalcitonin is elevated at 4.2 ng/mL, supporting bacterial etiology.

The patient is admitted and treated with IV ceftriaxone plus azithromycin (to cover typical and atypical pathogens). Blood cultures grow Streptococcus pneumoniae. The lobar consolidation pattern with air bronchograms reflects the pathologic stages of pneumococcal pneumonia: congestion, red hepatization, gray hepatization, and resolution. His fever resolves within 48 hours, and he is transitioned to oral antibiotics and discharged on hospital day 4 to complete a 5-day total antibiotic course.

### Key Learning Points
- Streptococcus pneumoniae is the most common cause of community-acquired pneumonia
- Classic presentation: acute onset, high fever with rigors, productive cough with rusty sputum, pleuritic pain
- Physical findings of consolidation: dullness, increased fremitus, bronchial breath sounds, egophony
- CURB-65 score guides disposition: 0-1 outpatient, 2 consider admission, 3+ inpatient/ICU
- Lobar consolidation with air bronchograms is classic radiographic finding

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## Case 2: Pulmonary Tuberculosis with Cavitary Disease

### Case Presentation
A 45-year-old man originally from India, now living in the US for 2 years, presents with a 3-month history of persistent cough productive of blood-tinged sputum, unintentional weight loss of 15 pounds, night sweats that soak his bedclothes, and low-grade fevers. He works in a homeless shelter. On examination, he appears cachectic. Vital signs show temperature 38.2C, heart rate 88 bpm, and oxygen saturation 95% on room air. Chest examination reveals decreased breath sounds and crackles in the right upper lobe.

Chest X-ray shows a right upper lobe cavitary lesion with surrounding infiltrate and no hilar lymphadenopathy. This pattern of apical cavitary disease is classic for reactivation tuberculosis, in contrast to primary TB which typically affects the middle/lower lobes with prominent hilar adenopathy.

Three sputum samples are collected for acid-fast bacilli (AFB) smear and culture. AFB smear is positive (3+), indicating high bacillary load and high infectivity. GeneXpert MTB/RIF (nucleic acid amplification test) confirms Mycobacterium tuberculosis without rifampin resistance, providing rapid results within hours while awaiting culture (which takes weeks).

The pathophysiology of reactivation TB involves breakdown of previously contained granulomas, often in the oxygen-rich lung apices where M. tuberculosis thrives. The cavities form from caseous necrosis liquefying and draining into airways, creating an oxygen-rich environment for massive bacterial proliferation. Cavitary disease is highly infectious.

The patient is placed in airborne isolation (negative-pressure room, N95 respirators for healthcare workers). He is started on RIPE therapy: Rifampin, Isoniazid, Pyrazinamide, and Ethambutol for 2 months (intensive phase), followed by Rifampin and Isoniazid for 4 months (continuation phase). Pyridoxine (vitamin B6) is given to prevent isoniazid-induced peripheral neuropathy. Public health is notified for contact investigation. After three consecutive negative AFB smears and clinical improvement, he is discharged on directly observed therapy (DOT) to ensure adherence.

### Key Learning Points
- Reactivation TB classically presents with upper lobe cavitary disease, constitutional symptoms (fever, night sweats, weight loss)
- Primary TB differs: middle/lower lobe disease with hilar lymphadenopathy in recently infected patients
- Cavitary disease indicates high bacillary load and high infectivity - requires airborne isolation
- Standard treatment: RIPE (Rifampin, Isoniazid, Pyrazinamide, Ethambutol) for 2 months, then RI for 4 months
- GeneXpert provides rapid TB confirmation and rifampin resistance detection

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## Case 3: Pneumocystis jirovecii Pneumonia in HIV/AIDS

### Case Presentation
A 34-year-old man with recently diagnosed HIV infection (not yet on antiretroviral therapy) presents with 3 weeks of progressive dyspnea, dry cough, and low-grade fevers. His CD4 count on diagnosis 1 month ago was 85 cells/microliter. On examination, he is tachypneic with respiratory rate 28/min. Oxygen saturation is 85% on room air, improving to only 90% on 6 L/min nasal cannula. Lung examination is surprisingly unremarkable despite his significant hypoxemia.

Chest X-ray shows bilateral diffuse interstitial infiltrates with a "ground-glass" pattern. CT chest confirms bilateral ground-glass opacities with characteristic sparing of the lung apices and periphery. LDH is markedly elevated at 580 U/L. Arterial blood gas on room air shows pH 7.46, PaCO2 30 mmHg, PaO2 52 mmHg with A-a gradient of 52 mmHg.

The clinical picture is classic for Pneumocystis jirovecii pneumonia (PCP): subacute onset over weeks, dry cough, exertional dyspnea, relatively normal lung exam despite severe hypoxemia, bilateral ground-glass infiltrates, and elevated LDH. PCP occurs almost exclusively in immunocompromised patients, particularly those with AIDS and CD4 less than 200 cells/microliter.

Induced sputum is obtained and tested with direct fluorescent antibody (DFA) staining, revealing P. jirovecii cysts. Bronchoscopy with BAL would be performed if induced sputum were negative, given the high clinical suspicion.

Treatment is initiated with high-dose trimethoprim-sulfamethoxazole (TMP-SMX). Given severe hypoxemia (PaO2 less than 70 mmHg on room air), adjunctive corticosteroids (prednisone) are added - this reduces mortality in moderate-to-severe PCP by blunting the inflammatory response as organisms die. He improves over 2 weeks and is discharged on TMP-SMX prophylaxis (to prevent recurrence while CD4 remains low) and antiretroviral therapy is initiated. Secondary prophylaxis continues until CD4 rises above 200 for at least 3 months.

### Key Learning Points
- PCP presents subacutely (weeks) with dyspnea, dry cough, and hypoxemia disproportionate to exam findings
- Occurs in immunocompromised patients, especially AIDS with CD4 less than 200
- Chest imaging shows bilateral ground-glass infiltrates; LDH is typically elevated
- TMP-SMX is treatment of choice; adjunctive corticosteroids if PaO2 less than 70 mmHg on room air
- Primary prophylaxis with TMP-SMX indicated when CD4 less than 200