Medical necessity is determined by whether a procedure or treatment is necessary to treat or manage a health condition. Removing excess skin after significant weight loss from a gastric bypass often meets medical necessity criteria because excess skin can lead to physical complications, such as infections, rashes, and mobility issues. Insurance companies are more likely to cover this procedure when it's needed to alleviate health issues rather than for cosmetic purposes.

A. Speech therapy for a lisp: Typically, therapy for minor speech impediments like a lisp may not be deemed medically necessary unless it severely affects communication or daily functioning.

B. Tummy tuck after a pregnancy: This procedure is generally classified as cosmetic and not medically necessary, as it is often done to improve appearance rather than address a health condition.

C. Second rhinoplasty for a smaller nose: This would likely be considered elective and cosmetic, especially if it is solely for aesthetic preference without any health-related issues.

Thus, the correct answer is D. Removing excess skin in losing weight from a gastric bypass, as it can be essential for physical health and quality of life.