During a parasitology practical, a medical student observes different morphological forms of Entamoeba histolytica under the microscope. Which is the most common form found in the stool of a patient with acute amoebic dysentery?

Explanation

## Morphological Forms of Entamoeba histolytica in Stool **Key Point:** Trophozoites are the most common form found in stool during acute amoebic dysentery because active tissue invasion and mucosal ulceration cause rapid shedding of motile trophozoites. ### Morphological Forms and Their Occurrence | Form | Size (μm) | Motility | Stool Type | Frequency in Dysentery | | --- | --- | --- | --- | --- | | Trophozoite | 15–60 | Amoeboid | Liquid/bloody | **Most common (acute)** | | Precyst | 10–20 | Reduced | Transitional | Intermediate | | Cyst | 12–16 | None | Formed | Most common (chronic/asymptomatic) | | Metacyst | 12–16 | None | Formed | Rare in stool | ### Life Cycle and Form Appearance ```mermaid flowchart TD A[Ingestion of cyst]:::outcome --> B[Excystation in colon]:::action B --> C[Metacyst releases trophozoite]:::action C --> D{Intestinal conditions?}:::decision D -->|Acute dysentery<br/>liquid stool| E[Trophozoites multiply<br/>invade mucosa]:::action D -->|Chronic/formed stool| F[Encystation begins]:::action E --> G[Trophozoites shed<br/>in liquid stool]:::outcome F --> H[Cysts formed<br/>in stool]:::outcome G --> I[Transmission via<br/>contaminated water]:::action H --> I ``` **High-Yield:** In acute dysentery, trophozoites are shed in large numbers because: 1. Active mucosal invasion causes rapid cellular turnover 2. Liquid stool prevents encystation 3. Osmotic diarrhoea accelerates transit, preventing cyst formation **Clinical Pearl:** Cysts are more commonly found in chronic infection or asymptomatic carriers with formed stools. Trophozoites are fragile and die quickly in formed stools, which is why they are rarely seen in chronic cases. **Mnemonic:** **TROPHOZOITE in ACUTE, CYST in CHRONIC** — Trophozoites dominate acute dysentery; cysts dominate chronic/asymptomatic carriage. **Warning:** Do not confuse precyst with cyst — precyst is an intermediate stage with reduced motility but not yet fully mature, and is rarely observed in routine microscopy. [cite:Park 26e Ch 13; Robbins 10e Ch 8]