**Core Concept:**
The question is discussing the clinical presentation of a malnourished 3-year-old girl with low haemoglobin (anaemia) and signs of edema, rash, and worm infestation. The correct diagnosis requires understanding the clinical features and the types of worm infestations that can cause these symptoms.

**Why the Correct Answer is Right:**
In this scenario, the correct diagnosis is **Ascariasis** (Ascaris lumbricoides). Ascariasis is a type of roundworm infection caused by the parasite Ascaris lumbricoides, which primarily targets the small intestine, leading to symptoms such as malnutrition, anemia, and organomegaly. The specific manifestations mentioned in the question are characteristic of ascariasis:

1. Anaemia (low haemoglobin): Ascariasis can cause malabsorption of nutrients due to the worm's presence in the small intestine, leading to anemia.
2. Edema (oedema): The malabsorption of essential nutrients can result in hypoproteinemia and decreased oncotic pressure, causing fluid retention and edema, particularly in areas where the lymphatics are most abundant, such as the knees and ankles.
3. Rash (desquamative dermatitis): The immune response to Ascaris antigens can lead to a desquamative dermatitis, causing the characteristic rash on the knees, ankles, and elbows in this case.

**Why Each Wrong Option is Incorrect:**
A. Filariasis (Wuchereria bancrofti) is a mosquito-borne filarial infection causing elephantiasis (edema), but not the described rash and anaemia.
B. Hookworm infection (e.g., Necator americanus) typically presents with anemia, abdominal pain, and diarrhea but not the described rash and edema.
C. Trichuriasis (Trichuris trichiura) can cause anemia, but not the described rash and edema.
D. Strongyloides stercoralis is a soil-transmitted helminth infection, typically presenting with gastrointestinal symptoms (e.g., diarrhea) and not the described rash and edema.

**Clinical Pearls:**
1. The correct diagnosis of Ascariasis can be made by considering the specific clinical presentation of the disease, including the rash, edema and anaemia. These symptoms are seen in children mostly, as they are more susceptible to soil-transmitted helminth infections.
2. In endemic areas where Ascaris is common, it is crucial to consider this diagnosis in children presenting with the described symptoms to avoid missing a treatable infectious cause of the patient's condition.