Which one of the following human diseases is correctly listed as a dominant autosomal disorder in classical genetics?

Introduction / Context:
Genetic disorders can be classified based on whether the disease causing allele is dominant or recessive and whether it is located on an autosome or a sex chromosome. Many exam questions require you to categorize well known disorders in this way. This question focuses on identifying a dominant autosomal disorder from a set of common examples that are often described in genetics chapters.

Given Data / Assumptions:

• The options list albinism, cystic fibrosis, phenylketonuria, and Alzheimer's disease.
• You are expected to know whether each disorder is typically autosomal recessive or autosomal dominant.
• We focus on standard textbook descriptions, especially of familial forms of disease.
• Sex linked disorders are not listed among the options, so we consider autosomal inheritance only.

Concept / Approach:
Albinism is usually cited as an autosomal recessive disorder affecting melanin pigment synthesis. Cystic fibrosis is a classic autosomal recessive disorder that affects the lungs and digestive system due to abnormal chloride channels. Phenylketonuria is also commonly described as an autosomal recessive inborn error of metabolism caused by deficiency of the enzyme phenylalanine hydroxylase. Certain familial forms of Alzheimer's disease, particularly early onset types, are caused by mutations in autosomal genes such as those coding for amyloid precursor protein and presenilins and are inherited as dominant traits. Therefore, among the given options, Alzheimer's disease best fits the category of a dominant autosomal disorder.

Step-by-Step Solution:
Step 1: Recall that albinism typically requires two copies of the defective allele for the condition to be expressed, which defines it as autosomal recessive.Step 2: Recall that cystic fibrosis is a widely known autosomal recessive disorder, often used as a textbook example of such inheritance.Step 3: Recall that phenylketonuria is also autosomal recessive, with carriers being unaffected but able to pass the gene to offspring.Step 4: Recognize that certain hereditary forms of Alzheimer's disease, especially early onset familial cases, are associated with mutations in autosomal genes and show a dominant pattern of inheritance in affected families.Step 5: Compare each option with the definition of an autosomal dominant disorder, which is one where a single defective allele from either parent can cause the disease.Step 6: Conclude that among the given options, Alzheimer's disease is the example that can be classified as a dominant autosomal disorder.

Verification / Alternative check:
Textbooks on human genetics often classify disorders like Huntington's disease and certain forms of familial Alzheimer's disease as autosomal dominant, illustrated with pedigrees where the condition appears in every generation. In contrast, recessive disorders such as cystic fibrosis, albinism, and phenylketonuria are shown in pedigrees where the disease may skip generations and appear only when both parents are carriers. By matching these patterns with the disorders listed in the options, you can verify that the familial forms of Alzheimer's disease belong in the dominant autosomal category.

Why Other Options Are Wrong:
Option A is wrong because albinism is usually autosomal recessive, not dominant. Option B is wrong because cystic fibrosis is also autosomal recessive. Option C is wrong because phenylketonuria is another classic example of an autosomal recessive metabolic disorder. These disorders require two copies of the mutant allele for expression and therefore do not fit the definition of dominant inheritance.

Common Pitfalls:
Students may memorize long lists of genetic diseases without carefully noting dominant or recessive inheritance patterns, which leads to confusion in questions like this. Another pitfall is to think that because Alzheimer's disease is common in older adults, it must always be sporadic rather than inherited. In reality, while many cases are not clearly inherited, there are well documented familial forms that are autosomal dominant. For exam purposes, focus on the examples given in your genetics syllabus and remember that albinism, cystic fibrosis, and phenylketonuria are recessive, whereas certain familial Alzheimer's cases are dominant autosomal disorders.

Final Answer:
Alzheimer's disease, which can occur in certain families as a dominant autosomal disorder.