A 2-year-old girl from Delhi presents with acute bloody diarrhea (10 stools per day with blood and mucus), fever (38.5°C), and signs of mild-to-moderate dehydration. Stool is positive for fecal leukocytes. Which investigation is most appropriate to confirm the diagnosis?

Question

Accepted Answer

C. Stool culture and sensitivity on selective media (HE agar, XLD agar). ## Clinical Presentation Analysis

**Key Point:** The clinical triad of **bloody diarrhea + fever + fecal leukocytes** strongly suggests **invasive bacterial gastroenteritis** (not viral or toxin-mediated).

## Differential Diagnosis of Acute Diarrhea

| Feature | Viral (Rotavirus) | Toxin-mediated (V. cholerae) | Invasive Bacterial (Shigella, Salmonella, EAEC) |
|---|---|---|---|
| **Stool type** | Watery, mucoid | Watery ("rice-water") | **Bloody, mucoid** |
| **Fever** | Low-grade | Absent | **Present** |
| **Fecal leukocytes** | Absent/rare | Absent | **Present** |
| **Tenesmus** | No | No | **Yes** |
| **Investigation of choice** | Antigen detection | Stool culture (enriched media) | **Stool culture (selective media)** |

## Why Stool Culture on Selective Media?

**High-Yield:** Fecal leukocytes + blood + fever = **invasive enteric bacteria** (Shigella, Salmonella, Campylobacter, EAEC). Selective media are required because:

1. **HE agar (Hektoen Enteric)** — inhibits gram-positive bacteria, allows Shigella and Salmonella (non-lactose fermenters) to grow as green colonies
2. **XLD agar (Xylose-Lysine-Deoxycholate)** — differentiates Shigella (red colonies) from Salmonella (yellow with black center)
3. **TCBS agar** — for Vibrio species (if cholera suspected)

**Clinical Pearl:** Shigella is the most common cause of **bloody diarrhea with fever and fecal leukocytes** in children in India. Stool culture on selective media is the gold standard for diagnosis and allows antimicrobial susceptibility testing (important for resistance patterns in India).

## Why Not Other Investigations?

**Mnemonic: FLAW** — Features that rule out viral diarrhea:
- **F**ecal leukocytes present (viral: absent)
- **L**arge volume bloody stools (viral: watery)
- **A**cute fever (viral: low-grade)
- **W**hite cells in stool (viral: none)

**Tip:** Antigen detection (rotavirus, norovirus) is for **watery diarrhea without blood or fever**. Bloody diarrhea = bacterial until proven otherwise.

Answer

A. Stool antigen detection for rotavirus and norovirus

Answer

B. Abdominal ultrasound for intussusception

Answer

D. Procalcitonin level

A 2-year-old girl from Delhi presents with acute bloody diarrhea (10 stools per day with blood and mucus), fever (38.5°C), and signs of mild-to-moderate dehydration. Stool is positive for fecal leukocytes. Which investigation is most appropriate to confirm the diagnosis?

Explanation

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