**Core Concept**
Rickets is a disease caused by vitamin D deficiency, leading to softening of bones in infants and children. It results from impaired calcium and phosphate absorption, affecting bone mineralization and growth.

**Why the Correct Answer is Right**
Vitamin D plays a crucial role in maintaining normal calcium and phosphate levels by stimulating their absorption in the gut. In rickets, vitamin D deficiency or resistance impairs this process, leading to hypocalcemia and hypophosphatemia. These electrolyte imbalances disrupt bone mineralization, causing softening and deformation of the bones.

**Why Each Wrong Option is Incorrect**
**Option A:** While growth retardation can be a feature of rickets, it is not a primary manifestation. Growth plates are affected, but the primary concern is bone deformity and mineralization defects.
**Option B:** Muscle weakness is a common feature of rickets due to hypocalcemia, which affects neuromuscular transmission. This option is a plausible manifestation.
**Option C:** Rickets is primarily a disease of bone mineralization and growth, rather than joint inflammation. While joints may be affected, this is not a primary feature.
**Option D:** Hypophosphatemia is a key feature of rickets, resulting from impaired phosphate absorption. This option is a correct manifestation.

**Clinical Pearl / High-Yield Fact**
Rickets is often associated with nutritional deficiencies, especially in populations with limited access to vitamin D-rich foods or sunlight. Breastfed infants are at higher risk if their mothers have vitamin D deficiency.

**Correct Answer:** C