**Core Concept**
In pediatrics, iron deficiency anemia (IDA) is a common cause of weakness in children. Hookworm infection is a leading cause of IDA worldwide. Hookworms attach to the intestinal mucosa, causing chronic blood loss and leading to iron deficiency.

**Why the Correct Answer is Right**
Hookworms (Ancylostoma duodenale and Necator americanus) infect humans through contaminated soil, causing intestinal infestation. The worms attach to the intestinal mucosa using their buccal capsule, which contains teeth-like structures. This attachment leads to chronic blood loss and iron deficiency anemia. The child's symptoms, including weakness, joint swelling, and bleeding spots, are consistent with hookworm infestation.

**Why Each Wrong Option is Incorrect**
* **Option B:** Roundworm (Toxocara spp.) infection can cause gastrointestinal symptoms, but it is not typically associated with iron deficiency anemia.
* **Option C:** Whipworm (Trichuris trichiura) infection can cause gastrointestinal symptoms, but it is not typically associated with iron deficiency anemia.
* **Option D:** Pinworm (Enterobius vermicularis) infection is a common cause of perianal itching, but it is not typically associated with iron deficiency anemia.

**Clinical Pearl / High-Yield Fact**
Hookworm infestation is a common cause of iron deficiency anemia in children, especially in developing countries. A single hookworm can cause up to 1 mL of blood loss per day, leading to significant iron deficiency over time.

**✓ Correct Answer: A. Hook worm**