**Core Concept**
The question is testing the student's knowledge of intestinal parasites that cause malabsorption syndrome. Malabsorption syndrome is a condition where the body is unable to properly absorb nutrients from food, leading to deficiencies and weight loss. Certain parasites can infect the small intestine and cause damage to the villi, leading to malabsorption.

**Why the Correct Answer is Right**
The correct answer is Giardia lamblia, a protozoan parasite that infects the small intestine and causes giardiasis. Giardia lamblia attaches to the intestinal mucosa and causes inflammation and damage to the villi, leading to malabsorption of fats, carbohydrates, and proteins. This results in symptoms such as diarrhea, abdominal cramps, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Ascaris lumbricoides, a roundworm parasite, can cause intestinal obstruction and respiratory symptoms, but it is not typically associated with severe malabsorption syndrome.
**Option B:** Trichuris trichiura, a whipworm parasite, can cause intestinal bleeding and anemia, but it is not typically associated with severe malabsorption syndrome.
**Option C:** Strongyloides stercoralis, a roundworm parasite, can cause intestinal symptoms such as diarrhea and abdominal pain, but it is not typically associated with severe malabsorption syndrome.

**Clinical Pearl / High-Yield Fact**
Giardiasis can be treated with metronidazole or albendazole, and it is essential to treat both the patient and their close contacts to prevent reinfection.

**Correct Answer: D. Giardia lamblia**