**Core Concept**
Hookworm infestation is a type of parasitic infection caused by Ancylostoma duodenale or Necator americanus, characterized by blood loss leading to anemia, particularly in children and individuals with inadequate nutrition. The infection is often associated with iron-deficiency anemia, growth retardation, and impaired cognitive development.

**Why the Correct Answer is Right**
Hookworm infestation leads to chronic blood loss, which results in iron-deficiency anemia. In this case, the girl's hemoglobin level of 5g/dl indicates severe anemia. The edema over her knees and ankles is likely due to protein deficiency, a common consequence of hookworm infestation. The discrete rash on her knees, ankles, and elbows may be indicative of cutaneous larva migrans, a skin manifestation associated with hookworm infection. The parasite's larvae penetrate the skin, causing an intense allergic reaction.

**Why Each Wrong Option is Incorrect**
**Option B:** Roundworm (Toxocara spp.) infection typically presents with gastrointestinal symptoms such as abdominal pain, diarrhea, and weight loss, but it is less likely to cause severe anemia or cutaneous manifestations.

**Option C:** Whipworm (Trichuris trichiura) infection can cause iron-deficiency anemia, but it is less common in children and typically presents with rectal prolapse, diarrhea, and abdominal pain.

**Option D:** Pinworm (Enterobius vermicularis) infection primarily causes anal itching and is not typically associated with severe anemia, edema, or cutaneous manifestations.

**Clinical Pearl / High-Yield Fact**
Hookworm infestation can be prevented by wearing shoes when walking barefoot, especially in areas contaminated with human feces. Regular deworming programs can also help control the spread of hookworm infection in endemic areas.

**✓ Correct Answer: A. Hook worm**