**Core Concept**
Osteopetrosis, also known as marble bone disease, is a rare, inherited disorder characterized by an increase in bone density due to impaired osteoclast function. This leads to a range of clinical manifestations, including skeletal deformities and complications.

**Why the Correct Answer is Right**
Option 1: Growth retardation is TRUE. Impaired osteoclast function leads to a decrease in bone resorption, which can result in an accumulation of dense bone tissue, causing growth retardation in children. Option 2: Mental retardation is FALSE. While osteopetrosis can have multiple systemic complications, mental retardation is not a typical feature of the disease. Option 3: Multiple fractures are FALSE. Due to the increased bone density, osteopetrosis is often associated with an increased risk of fractures, but it is not typically characterized by multiple fractures. Option 4: Cataract is TRUE. Osteopetrosis can be associated with a range of systemic complications, including cataracts, which are thought to be caused by the abnormal bone formation and accumulation of calcium. Option 5: Hepatosplenomegaly is TRUE. Impaired osteoclast function can lead to an accumulation of iron and copper in the liver and spleen, resulting in hepatosplenomegaly.

**Why Each Wrong Option is Incorrect**
**Option A:** Growth retardation is not a primary feature of osteopetrosis, although it may occur in some cases.
**Option B:** Mental retardation is not a typical feature of osteopetrosis, and its presence would suggest an underlying genetic or metabolic disorder.
**Option C:** Multiple fractures are not characteristic of osteopetrosis, which is often associated with an increased risk of fractures due to the increased bone density.
**Option D:** Cataract is not a universal feature of osteopetrosis, although it can occur in some cases.

**Clinical Pearl / High-Yield Fact**
Osteopetrosis is a rare, inherited disorder that can be associated with a range of systemic complications, including cataracts, hepatosplenomegaly, and an increased risk of fractures.

**Correct Answer:** D. All the statements are true.