**Core Concept**
Metal denture bases are used in dentistry to provide support and structure to prosthetic teeth. They are often preferred for patients with significant bone loss or those who require a high level of durability. However, metal denture bases have several limitations, including their impact on the surrounding tissues and the patient's overall health.

**Why the Correct Answer is Right**
The principal disadvantage of metal denture bases is their potential to cause tissue irritation and inflammation. This is due to the metal's reactivity with the surrounding tissues, leading to the release of ions that can cause an immune response. The metal denture base can also cause galvanic shock, which is a sudden release of ions that can cause pain and discomfort for the patient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while metal denture bases can be expensive, they are not the principal disadvantage. The cost of metal denture bases is a consideration for patients, but it is not the primary concern.
**Option B:** This option is incorrect because metal denture bases are not typically associated with allergic reactions. While some patients may be allergic to certain metals, this is not the principal disadvantage of metal denture bases.
**Option C:** This option is incorrect because metal denture bases are not typically associated with poor fit. In fact, metal denture bases are often preferred for their durability and ability to provide a precise fit.

**Clinical Pearl / High-Yield Fact**
It's essential to note that metal denture bases can cause tissue irritation and inflammation, especially if they are not properly fitted or if the patient has a history of metal allergies. To minimize these risks, dentists should carefully select the type of metal used and ensure that the denture base is properly fitted and maintained.

**Correct Answer:** D. Tissue irritation and inflammation.