**Core Concept**
Autosomal recessive (AR) inheritance involves two copies of a mutated gene (one from each parent) for disease expression. Affected individuals have clinically normal parents who are carriers, and the disorder affects both males and females equally due to equal gene transmission from both sexes.

**Why the Correct Answer is Right**
In autosomal recessive disorders, both parents are typically heterozygous carriers (clinically normal), and each child has a 25% chance of inheriting two mutant alleles and being affected. Since the gene is on an autosome, not a sex chromosome, males and females are equally affected. This pattern matches the clinical scenario described: unaffected parents, siblings affected equally, no sex bias.

**Why Each Wrong Option is Incorrect**
Option A: AD (Autosomal dominant) requires only one mutant allele; affected parents are usually present, which contradicts "clinically normal parents."
Option C: XLD (X-linked dominant) affects both sexes but usually shows higher male involvement and often has affected parents, which is not the case here.
Option D: XLR (X-linked recessive) primarily affects males and is usually seen in male-to-male transmission; females are rarely affected or are carriers, violating the "equal male-female proportion" rule.

**Clinical Pearl / High-Yield Fact**
In AR disorders, parents are unaffected carriers, and siblings are affected in equal numbers regardless of sex—this is a classic clue to distinguish AR from X-linked or dominant patterns.

✓ Correct Answer: B. AR