**Core Concept**
The patient's presentation of recurrent bulky stools and abdominal pain, accompanied by growth failure and pallor, suggests a malabsorptive disorder. This condition may be due to an underlying problem with the small intestine, such as a deficiency in disaccharidase enzymes or a structural abnormality.

**Why the Correct Answer is Right**
The correct answer is a ** Sweat Test**, which is used to diagnose cystic fibrosis (CF). CF is a genetic disorder that affects the respiratory, digestive, and reproductive systems. It is caused by mutations in the CFTR gene, leading to thick, sticky mucus production in the body. The sweat test detects elevated levels of chloride ions in sweat, a hallmark of CF. This condition can cause malabsorption of fats and fat-soluble vitamins due to pancreatic insufficiency, resulting in bulky, greasy stools and abdominal pain.

**Why Each Wrong Option is Incorrect**
**Option A:** A **Gastrointestinal Endoscopy** may be useful in diagnosing structural abnormalities or inflammation in the gastrointestinal tract, but it is not the most appropriate investigation for this patient's presentation.

**Option B:** A **24-hour pH Study** is used to diagnose gastroesophageal reflux disease (GERD) and is not relevant to this patient's symptoms of malabsorption.

**Option C:** A **Serum Vitamin B12 and Folate Levels** test may be useful in diagnosing malabsorptive disorders, but it is not the most specific investigation for this patient's presentation.

**Clinical Pearl / High-Yield Fact**
Cystic fibrosis is a multisystem disease that can affect multiple organs, including the lungs, pancreas, liver, kidneys, and reproductive system. Early diagnosis and treatment are crucial to prevent long-term complications and improve quality of life.

**Correct Answer:** A. Sweat Test