**Core Concept**
The patient's symptoms suggest an issue related to hemolysis, specifically the breakdown of red blood cells, which leads to the release of hemoglobin into the urine, causing it to darken upon standing.

**Why the Correct Answer is Right**
The patient's symptoms, including dark spots in the sclera (due to increased bilirubin levels) and blackening of urine on standing (due to hemoglobinuria), are classic signs of hemolytic anemia. This condition is often caused by an enzyme deficiency, such as glucose-6-phosphate dehydrogenase (G6PD) deficiency. G6PD is an enzyme that protects red blood cells from oxidative damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the patient's symptoms, as it does not explain the dark spots in the sclera or blackening of urine.
* **Option B:** This option may be a distractor, but it does not directly relate to the patient's symptoms. However, it's worth noting that certain infections can cause hemolytic anemia, but the patient's presentation is more suggestive of a congenital enzyme deficiency.
* **Option C:** This option is incorrect because it does not account for the dark spots in the sclera or the specific mechanism of hemolysis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hemolytic anemia can be caused by various factors, including enzyme deficiencies, infections, and autoimmune disorders. A thorough investigation, including a complete blood count and specific enzyme assays, is necessary to determine the underlying cause.

**Correct Answer: D. Alkaptonuria**