**Core Concept**
Influenza viruses exhibit two primary mechanisms of antigenic variation: antigenic shift and antigenic drift. Antigenic shift is a significant change in the viral genome, resulting in the creation of new subtypes, while antigenic drift is a gradual change in the viral surface proteins, leading to the emergence of new strains.

**Why the Correct Answer is Right**
Antigenic shift is a rare event that occurs through the reassortment of genetic material from different influenza viruses, typically between animal and human hosts. This process results in the creation of a new subtype, which can lead to a pandemic. In contrast, antigenic drift is a continuous process that involves the accumulation of point mutations in the viral genome, resulting in the emergence of new strains. However, not all types of influenza viruses exhibit antigenic shift; only type A viruses are capable of reassorting their genetic material.

**Why Each Wrong Option is Incorrect**
**Option B:** This statement is incorrect because all types of influenza viruses, including type A, exhibit antigenic drift, not just type A. Antigenic drift is a continuous process that involves the accumulation of point mutations in the viral genome.
**Option C:** This statement is incorrect because antigenic drift is not the accumulation of point mutations, but rather the gradual change in the viral surface proteins due to these mutations. While point mutations are involved in antigenic drift, the term "accumulation of point mutations" is not a precise description of the process.
**Option D:** This statement is incorrect because option A is indeed not true regarding influenza virus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that influenza viruses are classified into three types: A, B, and C. Type A viruses are the primary cause of seasonal epidemics and pandemics, while type B viruses cause seasonal epidemics, and type C viruses typically cause mild infections.

✓ Correct Answer: A. All types exhibit antigenic shift