Draw a rectangle behind tcolorbox

Question

I'd like to draw a rounded rectangle behind an existing tcolorbox. The result should look like this:

That's what I've got so far:

\documentclass{book}

\usepackage[many]{tcolorbox}
\usetikzlibrary{calc}
\definecolor{mp}{RGB}{240,240,240} % Farbe für Nummerierung

\tcbset{ % Rahmen
    colback=white,
  arc=20pt,
  freelance,
  frame code={
    \draw[rounded corners=20pt,color=mp, line width=1pt]
      (frame.south west) -- 
      (frame.north west) -- 
      (frame.north east) --
      (frame.south east) -- cycle;
  }
}

\begin{document}

Wert
\begin{tcolorbox}   
Erwachsen \\
8,9 - 10,1 mg/dL
2,25 - 2,75 mmol/L
Kinder höher
\end{tcolorbox}

\end{document}

Answer 1

One option using interior titled code to first draw the gray filled rounded rectangle for the title and then the rounded rectangle for the contents:

\documentclass{book}

\usepackage[many]{tcolorbox}
\usetikzlibrary{calc}
\definecolor{mp}{RGB}{240,240,240} % Farbe für Nummerierung

\tcbset{ % Rahmen
  arc=20pt,
  outer arc=20pt,
  title=Wert,
  coltitle=black,
  enhanced,
  freelance,
  frame code={},
  interior titled code={
    \fill[mp,rounded corners=12pt]
      ([xshift=-1pt,yshift=-10pt]interior.west)
      rectangle
      ([xshift=50pt,yshift=3pt]title.north west); 
    \draw[rounded corners=20pt,color=mp, line width=1pt,fill=white]
      (frame.south west) -- 
      (title.south west) -- 
      (title.south east) --
      (frame.south east) -- cycle;
  },
}

\begin{document}

\begin{tcolorbox}   
Erwachsen \\
8,9 - 10,1 mg/dL
2,25 - 2,75 mmol/L
Kinder höher
\end{tcolorbox}

\end{document}

The definition for the \wert command then might be something like

\documentclass{book}
\usepackage[many]{tcolorbox}
\usetikzlibrary{calc}
\definecolor{mp}{RGB}{240,240,240} % Farbe für Nummerierung

\newcommand\wert[1]{%
\begin{tcolorbox}[   
  arc=20pt,
  outer arc=20pt,
  title=Wert,
  coltitle=black,
  enhanced,
  freelance,
  frame code={},
  interior titled code={
    \fill[mp,rounded corners=12pt]
      ([xshift=-1pt,yshift=-10pt]interior.west)
      rectangle
      ([xshift=50pt,yshift=3pt]title.north west); 
    \draw[rounded corners=20pt,color=mp, line width=1pt,fill=white]
      (frame.south west) -- 
      (title.south west) -- 
      (title.south east) --
      (frame.south east) -- cycle;
  }
]
#1
\end{tcolorbox}%
}

\begin{document}

\wert{Erwachsen \\
8,9 - 10,1 mg/dL
2,25 - 2,75 mmol/L
Kinder höher}

\end{document}

If the title for the boxes must be variable, a simple modification for the above code (introducing an optional or mandatory argument for the tcolorbox) will suffice.