**Core Concept:**

The question is discussing a clinical scenario involving an infant presenting with symptoms and laboratory findings suggestive of a genetic disorder. The scenario describes a series of events that unfold over a week, starting with vomiting and diarrhea, followed by jaundice, fever, and hepatomegaly. The abnormal urine reducing substances and positive blood culture for Escherichia coli (E. coli) further support the diagnosis.

**Why the Correct Answer is Right:**

The correct answer is **G6PD deficiency (Glucose-6-Phosphate Dehydrogenase Deficiency)**. Glucose-6-Phosphate Dehydrogenase (G6PD) is an enzyme involved in the oxidative pentose phosphate pathway, which is crucial for maintaining the balance between the production and consumption of reactive oxygen species (ROS) in red blood cells (RBCs). In this case, an infant with G6PD deficiency is prone to oxidative stress due to the inability to handle high levels of certain drugs, foods, or infections, which can cause hemolysis (destruction of RBCs) and the clinical presentation described.

**Why Each Wrong Option is Incorrect:**

Let's discuss why the other options are incorrect:

A. **Galactosemia (Galactose Metabolism Disorders):** Galactosemia is a group of disorders caused by deficiency in enzymes involved in galactose catabolism, leading to abnormal galactose levels. The clinical presentation in this scenario is different from G6PD deficiency.

B. **Sickle Cell Disease:** Sickle cell disease is a genetic disorder characterized by abnormal hemoglobin, causing sickle-shaped RBCs and various complications, including jaundice, anemia, and organ damage. Although jaundice can be present in sickle cell disease, the other symptoms (vomiting, diarrhea, hepatomegaly, and fever) are not typical of this condition.

C. **Phenylketonuria (PKU):** PKU is a disorder due to deficiency of phenylalanine hydroxylase enzyme, leading to elevated phenylalanine levels. The clinical presentation is different from G6PD deficiency.

D. **Hemolytic anemia:** Hemolytic anemia is a condition characterized by rapid destruction of RBCs, leading to anemia. However, the specific clinical presentation described (vomiting, diarrhea, jaundice, hepatomegaly, and fever) differs from hemolytic anemia.

**Clinical Pearl:**

G6PD deficiency is a common X-linked genetic disorder affecting males predominantly. Glucose-6-phosphate dehydrogenase is an essential enzyme in the pentose phosphate pathway, responsible for producing NADPH and maintaining the balance of reactive oxygen species (ROS) in red blood cells (RBCs). An infant with G6PD deficiency is susceptible to oxidative stress, leading to hemolysis (destruction of RBCs) when exposed to certain drugs (e.g., fava beans), infections, or foods (e.g., fava beans).

**Correct Answer:**

The correct answer is G6PD deficiency. The enzyme deficiency leads to increased oxidative stress when exposed to