**Core Concept**
The patient's presentation is suggestive of a metabolic disorder characterized by abnormal pigmentation, calcification of intervertebral discs, and distinctive urinary findings. This condition is caused by an accumulation of a specific compound due to an enzymatic deficiency.

**Why the Correct Answer is Right**
The patient's symptoms, particularly the progressive palmoplantar pigmentation and calcification of the IV disc, are indicative of alkaptonuria. Alkaptonuria is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2-dioxygenase (HGD), an enzyme involved in the breakdown of tyrosine and phenylalanine. The accumulation of homogentisic acid (HGA) leads to its oxidation, resulting in the formation of a pigment called melanin. This pigment is responsible for the characteristic darkening of the urine, which turns greenish-brown upon addition of Benedict's reagent and blue-black upon oxidation. The greenish-brown precipitate is due to the formation of a compound called benedict's acid, which is an oxidation product of HGA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the patient's symptoms or the distinctive urinary findings described.
**Option B:** This option is incorrect because it is a different condition that affects the metabolism of another amino acid and does not cause the specific symptoms or urinary findings described in the patient's case.
**Option C:** This option is incorrect because it is a different condition that affects the metabolism of another compound and does not cause the specific symptoms or urinary findings described in the patient's case.

**Clinical Pearl / High-Yield Fact**
Alkaptonuria is a rare genetic disorder that can lead to significant morbidity, including ochronotic arthropathy and cardiovascular disease. Early diagnosis and management are crucial to prevent long-term complications. The distinctive urinary findings in alkaptonuria can be used as a diagnostic clue in patients presenting with suspected metabolic disorders.

**Correct Answer: C. Alkaptonuria**