A 4-year-old girl from rural Maharashtra presents with a 3-month history of persistent cough, low-grade fever in the evenings, and failure to thrive. Her weight has plateaued for the past 4 months. On examination, she is thin, with mild hepatomegaly. Chest X-ray shows a right hilar opacity with a clear air bronchogram sign. Mantoux test (5 TU) is 18 mm induration at 48 hours. Sputum smear microscopy is negative. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Pediatric Pulmonary TB with Hilar Lymphadenopathy ### Key Clinical Features **Key Point:** The combination of persistent cough (>3 weeks), low-grade fever, failure to thrive, and hilar opacity with air bronchogram in a child from an endemic area is classic for primary pulmonary TB. **Clinical Pearl:** In pediatric TB, the **hilar lymphadenopathy** (not cavitation) is the hallmark radiological finding. The "air bronchogram" sign represents patent airways within consolidated lung tissue, typical of TB lymphadenitis compressing bronchi. ### Diagnostic Criteria Met | Feature | Finding | Significance | | --- | --- | --- | | **Age** | 4 years | Peak incidence of primary TB in children | | **Duration** | 3 months | Chronic course typical of TB | | **Constitutional symptoms** | Fever + FTT | TB granulomatous inflammation | | **Chest X-ray** | Hilar opacity + air bronchogram | Pathognomonic for TB lymphadenitis | | **Mantoux test** | 18 mm (≥5 mm is positive in children) | Confirms TB infection | | **Sputum smear** | Negative | Expected in primary TB (paucibacillary) | | **Hepatomegaly** | Present | TB dissemination/granulomatous response | ### Why Sputum Smear is Negative **High-Yield:** Pediatric TB is typically **paucibacillary** (low bacillary load) because: 1. Children rarely develop cavitary disease (which sheds organisms) 2. Primary TB involves lymphadenitis, not tissue destruction 3. Sputum smear positivity is seen in <10% of children with TB ### Mantoux Interpretation in Children **Mnemonic: MANTOUX CUT-OFF IN CHILDREN — "5-10-15 Rule"** - ≥5 mm = positive (if BCG-vaccinated or immunocompromised) - ≥10 mm = positive (if BCG-vaccinated, normal immunity) - ≥15 mm = positive (any child) This child's 18 mm is clearly positive. ### Next Steps in Management 1. **Confirm diagnosis:** Gastric aspirate AFB smear (higher yield than sputum in children), or GeneXpert MTB/RIF 2. **Investigate source:** Household TB contact tracing (look for smear-positive adult) 3. **Assess dissemination:** Abdominal ultrasound (for TB meningitis risk), CSF examination if any CNS signs 4. **Start anti-TB therapy:** 4-drug regimen (HRZE) for 2 months, then HR for 4 months (standard pediatric regimen) ### Differential Diagnosis Exclusion **Key Point:** The **air bronchogram sign** is the radiological clue that rules out simple bronchial obstruction. In asthma or bronchiectasis, you would see hyperinflation or bronchial wall thickening, not a discrete hilar mass with patent airways. [cite:Park 26e Ch 7 (Tuberculosis in Children)] [cite:Harrison 21e Ch 205 (Tuberculosis)]