**Core Concept**
Invasive amebiasis, caused by the protozoan parasite *Entamoeba histolytica*, requires a diagnostic approach that can differentiate it from other gastrointestinal infections. The investigation of choice should ideally detect the parasite, its antigens, or the associated pathology.

**Why the Correct Answer is Right**
The correct investigation of choice for invasive amebiasis is serological testing, specifically enzyme-linked immunosorbent assay (ELISA), for the detection of *E. histolytica* antigens in stool or serum. This method is highly sensitive and specific for invasive amebiasis, allowing for the differentiation from asymptomatic carriage or non-invasive infections. The detection of antigens in stool or serum correlates with the presence of the parasite in tissues and the severity of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Stool examination for ova and parasites (O&P) is not the investigation of choice for invasive amebiasis because it may not detect the parasite in all cases, especially in early stages of the disease or in cases with low parasite load.

**Option B:** Colonoscopy may be useful in detecting complications of invasive amebiasis, such as ulcers or strictures, but it is not the investigation of choice for the initial diagnosis.

**Clinical Pearl / High-Yield Fact**
Invasive amebiasis can present with a wide range of clinical manifestations, from asymptomatic carriage to fulminant colitis, and requires prompt diagnosis and treatment to prevent complications and mortality.

**Correct Answer:** C. Serological testing (ELISA) for *E. histolytica* antigens.