**Core Concept**
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive disorder affecting the enzyme G6PD, which plays a crucial role in protecting red blood cells (RBCs) from oxidative stress. This deficiency leads to hemolytic anemia.

**Why the Correct Answer is Right**
G6PD deficiency is an X-linked recessive disorder, meaning the mutated gene responsible for the condition is located on the X chromosome. This results in the condition predominantly affecting males, who have only one X chromosome. The enzyme level in RBCs is directly proportional to the age of the RBC, as younger RBCs have higher enzyme levels than older RBCs. The presence of bite cells (+) is a characteristic feature of G6PD deficiency, indicating that the RBCs have been subjected to oxidative stress. G6PD deficiency does not protect against kala-azar, which is a parasitic infection caused by Leishmania species.

**Why Each Wrong Option is Incorrect**
**Option A:** Autosomal dominant - This is incorrect because G6PD deficiency is an X-linked recessive disorder, not an autosomal dominant disorder.
**Option C:** Protects against kala-azar - This is incorrect because G6PD deficiency does not provide protection against kala-azar; in fact, individuals with G6PD deficiency may be at increased risk of severe kala-azar due to the oxidative stress caused by the parasite.
**Option D:** Enzyme level directly proportional to age of RBC - This is incorrect because the enzyme level in RBCs is actually inversely proportional to the age of the RBC, not directly proportional.

**Clinical Pearl / High-Yield Fact**
G6PD deficiency is a common cause of hemolytic anemia worldwide, particularly in individuals of African, Mediterranean, and Middle Eastern descent. A mnemonic to remember the key features of G6PD deficiency is "BITE": Bite cells, Increased susceptibility to oxidative stress, Tendency to develop hemolytic anemia, and Elevated levels in younger RBCs.

**Correct Answer:** B. Bite cell (+)