Okay, let's tackle this question. The patient is a 15-year-old girl with multiple skin nodules and light brown macules, along with family history. The key here is to figure out the inheritance pattern based on the symptoms and family history.

First, the clinical features: multiple nodules and macules, plus iris involvement. The presence of these symptoms over 10 years and family members affected suggests a genetic disorder. The question mentions a loss-of-function mutation. So, I need to think about genetic disorders that present with these skin and eye manifestations.

The most likely diagnosis here is Neurofibromatosis type 1 (NF1). NF1 is characterized by café-au-lait spots, multiple neurofibromas, and Lisch nodules in the iris. The criteria for NF1 include having at least two of the following: six or more café-au-lait spots, two or more neurofibromas, freckling in the axillae or groin, optic glioma, and family history. The patient has 12 café-au-lait spots and iris nodules, which fits.

Now, inheritance patterns. NF1 is an autosomal dominant disorder. That means a single copy of the mutated gene is sufficient to cause the disease. Since the patient has affected family members (sibling and parent), this supports an autosomal dominant pattern. The question mentions a loss-of-function mutation, which is common in autosomal dominant diseases where one non-functional allele is enough to cause the phenotype, even if the other allele is normal.

Looking at the options, the correct answer should be autosomal dominant. The other options like autosomal recessive, X-linked dominant, or recessive are less likely. For example, autosomal recessive would require both parents to be carriers, and only 25% chance of offspring being affected. The family history here shows a parent and sibling affected, which is more consistent with dominant inheritance. X-linked would present differently, especially in males, but the description here doesn't specify gender issues. The presence in both parent and child also points to dominant, as a parent with the mutation can pass it on to any child regardless of gender.

Clinical pearl: Remember that NF1 is autosomal dominant. The presence of multiple café-au-lait spots and iris nodules (Lisch nodules) are classic signs. Also, the diagnostic criteria for NF1 include specific numbers of these features. The loss-of-function mutation fits the dominant negative effect, where the mutant protein interferes with the normal function of the other allele.

So, the inheritance pattern here is autosomal dominant. The correct answer must be option B if the options include that. The other options don't fit the clinical and family history data provided.

**Core Concept**
This question tests understanding of **autosomal dominant inheritance patterns**, particularly in **Neurofibromatosis type 1 (NF1)**, a disorder characterized by café-au-lait macules, neurofibromas, and Lisch nodules. NF1 is caused by **loss-of-function mutations** in the **NF1 tumor suppressor gene**.

**Why the Correct Answer is Right**
The clinical features—**20 neurofibromas**, **12 café-au-lait macules**, and **pigmented iris nodules (Lisch nodules)**—are classic for **NF1**, which follows **autosomal dominant

✓ Correct Answer: A. Autosomal dominant