Final answer:
The best test for diagnosing Huntington's disease is the serum polymerase chain reaction (PCR), which detects the specific genetic mutation causing the disorder.
Step-by-step explanation:
The best test for diagnosing Huntington's disease is D. Serum polymerase chain reaction (PCR). Huntington's disease is a genetic disorder caused by a mutation in the gene that encodes the huntingtin protein, which results in a longer than normal sequence of amino acids (glutamine residues) at the N-terminus of the protein. Diagnosing this condition with certainty requires the identification of this specific genetic mutation. PCR is a molecular technique that amplifies specific segments of DNA, making it a highly effective tool for detecting the presence of the Huntington gene mutation.
While other options like karyotyping can reveal larger chromosomal abnormalities, it is not specific enough for Huntington's disease. Similarly, serum ceruloplasmin and urine porphobilinogens are related to other disorders, not Huntington's disease. The CSF assay for 14-3-3 proteinase inhibitor protein is more relevant to diagnosing prion diseases such as Creutzfeldt-Jakob disease, not Huntington's disease.