**Core Concept**
Cystinosis is a rare genetic disorder characterized by the accumulation of cystine within lysosomes due to a deficiency of the cystine transporter, cystinosin. This leads to cellular damage and organ dysfunction, primarily affecting the kidneys, eyes, and pancreas.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of cystinosis. Cystinosis is caused by mutations in the CTNS gene, which encodes the cystinosin protein responsible for transporting cystine out of lysosomes. Accumulation of cystine within lysosomes leads to the formation of cystine crystals, causing cellular damage and organ dysfunction. The correct answer will be related to the underlying mechanism of cystinosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystinosis is a lysosomal storage disorder caused by a deficiency of cystinosin. This statement is true.
**Option B:** Cystinosis is characterized by the accumulation of cystine within lysosomes. This statement is true.
**Option C:** Cystinosis is primarily treated with cysteamine therapy, which helps to reduce cystine accumulation within lysosomes. This statement is true.

**Clinical Pearl / High-Yield Fact**
A key feature of cystinosis is the presence of Fanconi syndrome, a disorder of the proximal renal tubules characterized by impaired reabsorption of glucose, amino acids, and phosphate.

**Correct Answer:** D