**Core Concept**
Krabbe’s disease is a rare, inherited disorder that affects the nervous system, characterized by a deficiency in the enzyme responsible for the breakdown of a specific fatty acid in the brain. This deficiency leads to the accumulation of toxic substances, causing progressive damage to the myelin sheath surrounding nerve fibers.

**Why the Correct Answer is Right**
Krabbe’s disease is caused by a deficiency of galactosylceramidase (also known as galactocerebrosidase), an enzyme that breaks down galactosylceramide, a fatty acid derivative. This enzyme deficiency leads to the accumulation of galactosylceramide, which is toxic to oligodendrocytes, the cells responsible for myelination. As a result, the myelin sheath is damaged, disrupting normal nerve function and leading to the characteristic symptoms of Krabbe’s disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the enzyme deficiency in Krabbe’s disease is not related to sphingomyelinase, which is the enzyme deficient in Niemann-Pick disease.
**Option B:** This option is incorrect because the enzyme deficiency in Krabbe’s disease is not related to alpha-Gal A, which is the enzyme deficient in Fabry disease.
**Option C:** This option is incorrect because the enzyme deficiency in Krabbe’s disease is not related to beta-Gal A, which is not a known enzyme deficiency in any disease.

**Clinical Pearl / High-Yield Fact**
Krabbe’s disease is a classic example of a lysosomal storage disorder, where a deficiency in a lysosomal enzyme leads to the accumulation of toxic substances and progressive damage to the nervous system. This highlights the importance of lysosomal function in maintaining normal cellular processes.

**Correct Answer:** D. Galactosylceramidase