American Board of Psychiatry and Neurology (ABPN) Practice Exam 2025 - Free ABPN Practice Questions and Study Guide

The correct diagnosis that might be assessed using serum PCR to evaluate trinucleotide repeats is Huntington's disease. This condition is associated with an expansion of the CAG repeat in the HTT gene, which is located on chromosome 4. Using polymerase chain reaction (PCR) allows for the amplification of specific DNA sequences, making it possible to detect the number of CAG repeats in an individual's genes.

Individuals with Huntington's disease typically have more than 35 CAG repeats, whereas those with normal alleles have fewer than this threshold. The identification of these repeats is crucial since it not only confirms the diagnosis but also has implications for counseling and management regarding the hereditary nature of the disease.

In contrast, the other conditions mentioned—bipolar disorder, anxiety disorders, and obsessive-compulsive disorder—do not involve identified trinucleotide repeat expansions as a pathophysiological mechanism. These mental health disorders have multifactorial etiologies primarily rooted in psychological, environmental, and other genetic factors, rather than specific molecular markers detectable by PCR related to trinucleotide repeats.