An 8-month-old baby girl had normal growth and development for the first few months, but then progressively deteriorated with deafness, blindness, atrophied muscle, inability to swallow, and seizures. Early on in the diagnosis of the child, it was noticed that a cherry red macula was present in both eyes. Considering the child in the above case, measurement of which one of the following would enable one to determine whether the mutation were in the hex A or hex B gene?

**Core Concept**
The child in the case description is likely suffering from a lysosomal storage disorder, specifically a form of mucopolysaccharidosis (MPS). This group of diseases is caused by the deficiency of enzymes required for the breakdown of glycosaminoglycans (GAGs) in lysosomes. The presence of a cherry red macula in both eyes is a characteristic feature of these disorders.

**Why the Correct Answer is Right**
The correct answer is based on the measurement of the size of the lysosome and the amount of stored material within it. Hex A and hex B are enzymes involved in the breakdown of GAGs. The size of the lysosome and the amount of stored material would be different depending on whether the mutation is in the hex A or hex B gene. **Measurement of the lysosomal volume and the amount of stored GAGs** would enable one to determine whether the mutation were in the hex A or hex B gene.

**Why Each Wrong Option is Incorrect**
* **Option A:** Measurement of the lysosomal volume is not a reliable method to determine the type of mutation.
* **Option B:** The amount of stored GAGs in the lysosome is not a direct measure of the type of mutation.
* **Option D:** The activity level of the hex A or hex B enzyme is not directly related to the measurement of lysosomal volume or stored material.

**Clinical Pearl / High-Yield Fact**
In patients with lysosomal storage disorders, the cherry red macula is caused by the accumulation of GAGs in the retinal pigment epithelium, leading to a characteristic red discoloration of the macula.

**Correct Answer: C. Measurement of the lysosomal volume and the amount of stored GAGs.**

✓ Correct Answer: C. Globoside