**Core Concept**
Klinefelter syndrome is a genetic condition in males characterized by the presence of one or more extra X chromosomes, resulting in an XXY or XYY karyotype. This condition is associated with hypogonadism, infertility, and an increased risk of certain medical complications.

**Why the Correct Answer is Right**
The diagnosis of Klinefelter syndrome is typically made through a combination of clinical evaluation and cytogenetic analysis. A karyotype test, which examines the individual's chromosomes, is the most definitive diagnostic tool. This test can identify the presence of extra X chromosomes, confirming the diagnosis of Klinefelter syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not specify a diagnostic method. While clinical evaluation can provide clues for the diagnosis of Klinefelter syndrome, it is not a specific diagnostic tool.

**Option B:**
This option is incorrect because ultrasound is not a definitive diagnostic tool for Klinefelter syndrome. While ultrasound may be used to evaluate testicular size and other physical characteristics, it cannot confirm the presence of extra X chromosomes.

**Option C:**
This option is incorrect because physical examination alone is not sufficient to diagnose Klinefelter syndrome. While individuals with Klinefelter syndrome may exhibit certain physical characteristics, such as tall stature and gynecomastia, a definitive diagnosis requires cytogenetic analysis.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that individuals with Klinefelter syndrome are at an increased risk of developing certain medical complications, including infertility, obesity, and autoimmune disorders. A high index of suspicion and early diagnosis are essential to prevent or manage these complications.

**Correct Answer:** D. Karyotype test.