**Core Concept:** Perthes disease is a childhood disorder characterized by avascular necrosis (death of bone tissue) of the femoral head, commonly affecting the hip joint. This condition results from a disruption in blood supply to the femoral head, leading to decreased nutrient supply and oxygen deprivation to the bone, eventually causing bone remodeling and deformity.

**Why the Correct Answer is Right:**
Perthes disease primarily affects children between the ages of 4 and 10 years, during a critical period of bone growth and development. The correct answer focuses on the critical factor in the etiology of Perthes disease: the disruption of blood supply to the femoral head. This disruption leads to decreased nutrient supply and oxygen deprivation to the bone, causing damage to the bone tissue and subsequent remodeling and deformity.

**Why Each Wrong Option is Incorrect:**
A. Limited joint motion (option A) is a consequence, not the cause, of Perthes disease. This option does not address the primary etiology of the condition.
B. Nutritional deficiency (option B) is a possible contributing factor but does not capture the primary cause, which is the disruption of blood supply.
C. Infection (option C) is an unrelated factor and does not address the core mechanism of Perthes disease.
D. Osteochondritis dissecans (option D) is a different condition characterized by detachment of a piece of bone and cartilage from a joint surface, not Perthes disease.

**Clinical Pearl / High-Yield Fact:**
Understanding the fundamental etiology of Perthes disease (disruption of blood supply) helps in distinguishing it from other hip joint disorders, guiding diagnosis, and management strategies.

**Correct Answer:** C. Disruption of blood supply to the femoral head is the primary cause of Perthes disease, which leads to avascular necrosis, bone remodeling, and deformity. This question specifically addresses the etiology of the disorder and helps students understand the critical factors contributing to the disease process.