Investigation of choice for invasive amoebiasis is:
**Core Concept**
Invasive amoebiasis, caused by *Entamoeba histolytica*, is a severe form of amoebiasis characterized by the invasion of intestinal or extraintestinal tissues. The diagnosis of invasive amoebiasis requires a sensitive and specific investigation to identify the amoeba in the tissues or its metabolic products.
**Why the Correct Answer is Right**
The investigation of choice for invasive amoebiasis is serological testing, specifically the indirect hemagglutination (IHA) test or the enzyme-linked immunosorbent assay (ELISA). These tests detect antibodies against *E. histolytica* in the patient's serum, which are produced in response to the infection. The IHA test is more sensitive and specific than the ELISA test. The detection of these antibodies is crucial for diagnosing invasive amoebiasis, especially in patients with extraintestinal manifestations.
**Why Each Wrong Option is Incorrect**
**Option A:** Stool examination is not the investigation of choice for invasive amoebiasis because it may not detect the amoeba in the tissues or its metabolic products. Stool examination is more useful for diagnosing intestinal amoebiasis.
**Option B:** Endoscopy with biopsy is not the investigation of choice for invasive amoebiasis because it is invasive and not widely available. While endoscopy with biopsy can be useful for diagnosing invasive amoebiasis, it is not the first-line investigation.
**Option C:** CT scan is not the investigation of choice for invasive amoebiasis because it is not specific for diagnosing amoebiasis. A CT scan can show the extent of the disease, but it does not provide a specific diagnosis.
**Clinical Pearl / High-Yield Fact**
Remember that invasive amoebiasis can present with non-specific symptoms such as fever, abdominal pain, and weight loss, making it challenging to diagnose. A high index of suspicion and prompt serological testing are essential for diagnosing invasive amoebiasis.
**Correct Answer:** C. CT scan is not the investigation of choice for invasive amoebiasis because it is not specific for diagnosing amoebiasis.