**Question:** All are true about Phthisis bulbi except

A. It is a rare condition.
B. It is caused by a deficiency in vitamin A.
C. It typically affects individuals over the age of 60.
D. It results in blindness due to optic nerve atrophy.

**Core Concept:**
Phthisis bulbi is a term used to describe the advanced stage of various ocular diseases characterized by progressive atrophy of the optic nerve and the resultant irreversible blindness. It is a rare condition, occurring in approximately 1 in 100,000 individuals worldwide. The primary cause of phthisis bulbi is usually due to a deficiency in vitamin A, which is essential for maintaining healthy ocular tissues. Vitamin A deficiency can result from malabsorption syndromes, malnutrition, or improper dietary habits.

**Why the Correct Answer is Right:**
Option B is incorrect because phthisis bulbi is primarily caused by a deficiency in vitamin A, not a deficiency in other vitamins or nutrients. While vitamin A deficiency can contribute to the development of phthisis bulbi, other factors may also play a role in the progression of the condition.

**Why Each Wrong Option is Incorrect:**
Option A is incorrect because phthisis bulbi can affect individuals of any age, not just those over 60 years old. It can occur in children, adolescents, and adults of any age group.

Option C is incorrect because phthisis bulbi can affect individuals of any age, not just those over 60 years old. It can occur in children, adolescents, and adults of any age group.

Option D is incorrect because the progression of phthisis bulbi is not solely due to optic nerve atrophy. Other factors contribute to the condition, such as vitamin A deficiency and the resulting atrophy of ocular tissues. Optic nerve atrophy is just one of the consequences of the underlying condition deteriorating the ocular tissues.

**Core Concept:**
Phthisis bulbi is a severe complication of various ocular diseases affecting the retina and optic nerve. It is characterized by progressive atrophy of the optic nerve and the optic disc, leading to irreversible blindness.

**Why the Correct Answer is Right:**
Option D is correct because the progression of phthisis bulbi is not solely due to optic nerve atrophy. The condition is primarily caused by a deficiency in vitamin A, which is essential for maintaining healthy ocular tissues. Vitamin A deficiency can result from malabsorption syndromes, malnutrition, or inadequate dietary habits. Optic nerve atrophy is one of the consequences of the underlying condition causing deterioration of ocular tissues.

**Why Each Wrong Option is Incorrect:**
Option A is incorrect because phthisis bulbi can affect individuals of any age group, not just older adults. It can occur in children, adolescents, and adults of any age group.

Option C is incorrect because phthisis bulbi can affect individuals of any age group, not just older adults. It can occur in children, adolescents, and adults of any age group.