diff --git a/src/components/settings.tsx b/src/components/settings.tsx
index 6fb7383..d885626 100644
--- a/src/components/settings.tsx
+++ b/src/components/settings.tsx
@@ -18,6 +18,7 @@ import { Button } from './ui/button';
 import { Tooltip, TooltipContent, TooltipTrigger } from './ui/tooltip';
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from './ui/select';
 import { FormNumericInput } from './ui/form-numeric-input';
+import { FormSelect } from './ui/form-select';
 import CollapsibleCardHeader from './ui/collapsible-card-header';
 import { Info } from 'lucide-react';
 import { getDefaultState } from '../constants/defaults';
@@ -37,6 +38,7 @@ const getDefaultsForTranslation = (pkParams: any, therapeuticRange: any, uiSetti
     yAxisMax: defaults.uiSettings.yAxisMax,
     therapeuticRangeMin: defaults.therapeuticRange.min,
     therapeuticRangeMax: defaults.therapeuticRange.max,
+    showDayTimeOnXAxis: defaults.uiSettings.showDayTimeOnXAxis,
 
     // PK Parameters
     damphHalfLife: defaults.pkParams.damph.halfLife,
@@ -52,6 +54,9 @@ const getDefaultsForTranslation = (pkParams: any, therapeuticRange: any, uiSetti
     fOral: defaults.pkParams.advanced.fOral,
     fOralPercent: String((parseFloat(defaults.pkParams.advanced.fOral) * 100).toFixed(1)),
     steadyStateDays: defaults.pkParams.advanced.steadyStateDays,
+    urinePh: defaults.pkParams.advanced.urinePh.mode,
+    ageGroup: defaults.pkParams.advanced.ageGroup?.preset || 'adult',
+    renalFunctionSeverity: defaults.pkParams.advanced.renalFunction?.severity || 'normal',
   };
 };
 
@@ -525,13 +530,13 @@ const Settings = ({
 
             <div className="space-y-2">
               <Label className="font-medium">{t('xAxisTimeFormat')}</Label>
-                <Select
+                <FormSelect
                   value={showDayTimeOnXAxis}
                   onValueChange={value => onUpdateUiSetting('showDayTimeOnXAxis', value)}
+                  showResetButton={true}
+                  defaultValue={defaultsForT.showDayTimeOnXAxis}
+                  triggerClassName="w-[360px]"
                 >
-                  <SelectTrigger className="w-[360px]">
-                    <SelectValue />
-                  </SelectTrigger>
                   <SelectContent>
                     <Tooltip>
                       <TooltipTrigger asChild>
@@ -564,7 +569,7 @@ const Settings = ({
                       </TooltipContent>
                     </Tooltip>
                   </SelectContent>
-                </Select>
+                </FormSelect>
             </div>
           </CardContent>
         )}
@@ -834,20 +839,20 @@ const Settings = ({
                   </TooltipContent>
                 </Tooltip>
               </div>
-              <Select
+              <FormSelect
                 value={pkParams.advanced.standardVd?.preset || 'adult'}
-                onValueChange={(value: 'adult' | 'child' | 'custom' | 'weight-based') => updateAdvanced('standardVd', 'preset', value)}
+                onValueChange={(value) => updateAdvanced('standardVd', 'preset', value as 'adult' | 'child' | 'custom' | 'weight-based')}
+                showResetButton={true}
+                defaultValue={defaultsForT.standardVdPreset}
+                triggerClassName="w-[360px]"
               >
-                <SelectTrigger className="w-[360px]">
-                  <SelectValue />
-                </SelectTrigger>
                 <SelectContent>
                   <SelectItem value="adult">{t('standardVdPresetAdult')}</SelectItem>
                   <SelectItem value="child">{t('standardVdPresetChild')}</SelectItem>
                   <SelectItem value="custom">{t('standardVdPresetCustom')}</SelectItem>
                   <SelectItem value="weight-based">{t('standardVdPresetWeightBased')}</SelectItem>
                 </SelectContent>
-              </Select>
+              </FormSelect>
               {pkParams.advanced.standardVd?.preset === 'weight-based' && (
                 <div className="ml-0 mt-2 p-2 bg-blue-50 dark:bg-blue-900/20 border border-blue-200 dark:border-blue-800 rounded text-xs text-blue-800 dark:text-blue-200">
                   ⓘ {t('weightBasedVdInfo')}
@@ -967,21 +972,21 @@ const Settings = ({
                   </Tooltip>
               </div>
               <div>
-                <Select
+                <FormSelect
                   value={pkParams.advanced.urinePh.mode}
-                  onValueChange={(value: 'normal' | 'acidic' | 'alkaline') =>
-                    updateAdvanced('urinePh', 'mode', value)
+                  onValueChange={(value) =>
+                    updateAdvanced('urinePh', 'mode', value as 'normal' | 'acidic' | 'alkaline')
                   }
+                  showResetButton={true}
+                  defaultValue={defaultsForT.urinePh}
+                  triggerClassName="w-[360px]"
                 >
-                  <SelectTrigger id="urinePHMode" className="w-[360px]">
-                    <SelectValue />
-                  </SelectTrigger>
                   <SelectContent>
                     <SelectItem value="normal">{t('urinePHModeNormal')}</SelectItem>
                     <SelectItem value="acidic">{t('urinePHModeAcidic')}</SelectItem>
                     <SelectItem value="alkaline">{t('urinePHModeAlkaline')}</SelectItem>
                   </SelectContent>
-                </Select>
+                </FormSelect>
               </div>
             </div>
 
@@ -1008,21 +1013,21 @@ const Settings = ({
                   </TooltipContent>
                 </Tooltip>
               </div>
-              <Select
+              <FormSelect
                 value={pkParams.advanced.ageGroup?.preset || 'adult'}
-                onValueChange={(value: 'child' | 'adult' | 'custom') => {
-                  updateAdvancedDirect('ageGroup', { preset: value });
+                onValueChange={(value) => {
+                  updateAdvancedDirect('ageGroup', { preset: value as 'child' | 'adult' | 'custom' });
                 }}
+                showResetButton={true}
+                defaultValue={defaultsForT.ageGroup}
+                triggerClassName="w-[360px]"
               >
-                <SelectTrigger className="w-[360px]">
-                  <SelectValue />
-                </SelectTrigger>
                 <SelectContent>
                   <SelectItem value="adult">{t('ageGroupAdult')}</SelectItem>
                   <SelectItem value="child">{t('ageGroupChild')}</SelectItem>
                   <SelectItem value="custom">{t('ageGroupCustom')}</SelectItem>
                 </SelectContent>
-              </Select>
+              </FormSelect>
             </div>
 
             <Separator className="my-4" />
@@ -1065,24 +1070,24 @@ const Settings = ({
                   <div className="flex items-center gap-2">
                     <Label className="text-sm font-medium">{t('renalFunctionSeverity')}</Label>
                   </div>
-                  <Select
+                  <FormSelect
                     value={pkParams.advanced.renalFunction?.severity || 'normal'}
-                    onValueChange={(value: 'normal' | 'mild' | 'severe') => {
+                    onValueChange={(value) => {
                       updateAdvancedDirect('renalFunction', {
                         enabled: true,
-                        severity: value
+                        severity: value as 'normal' | 'mild' | 'severe'
                       });
                     }}
+                    showResetButton={true}
+                    defaultValue={defaultsForT.renalFunctionSeverity}
+                    triggerClassName="w-full"
                   >
-                    <SelectTrigger className="w-full">
-                      <SelectValue />
-                    </SelectTrigger>
                     <SelectContent>
                       <SelectItem value="normal">{t('renalFunctionNormal')}</SelectItem>
                       <SelectItem value="mild">{t('renalFunctionMild')}</SelectItem>
                       <SelectItem value="severe">{t('renalFunctionSevere')}</SelectItem>
                     </SelectContent>
-                  </Select>
+                  </FormSelect>
                 </div>
               )}
             </div>
diff --git a/src/components/ui/form-select.tsx b/src/components/ui/form-select.tsx
new file mode 100644
index 0000000..25a6ac8
--- /dev/null
+++ b/src/components/ui/form-select.tsx
@@ -0,0 +1,64 @@
+/**
+ * Custom Form Component: Select with Reset Button
+ *
+ * A select/combobox field with an optional reset to default button.
+ * Built on top of shadcn/ui Select component.
+ *
+ * @author Andreas Weyer
+ * @license MIT
+ */
+
+import * as React from "react"
+import { RotateCcw } from "lucide-react"
+import { IconButtonWithTooltip } from "./icon-button-with-tooltip"
+import { Select, SelectTrigger, SelectValue, SelectContent } from "./select"
+import { cn } from "../../lib/utils"
+import { useTranslation } from "react-i18next"
+
+interface FormSelectProps {
+  value: string
+  onValueChange: (value: string) => void
+  showResetButton?: boolean
+  defaultValue?: string
+  children: React.ReactNode
+  triggerClassName?: string
+  placeholder?: string
+}
+
+export const FormSelect: React.FC<FormSelectProps> = ({
+  value,
+  onValueChange,
+  showResetButton = false,
+  defaultValue,
+  children,
+  triggerClassName,
+  placeholder,
+}) => {
+  const { t } = useTranslation()
+
+  return (
+    <div className="flex items-center gap-0">
+      <Select value={value} onValueChange={onValueChange}>
+        <SelectTrigger className={cn(
+          showResetButton && "rounded-r-none border-r-0 z-10",
+          triggerClassName
+        )}>
+          <SelectValue placeholder={placeholder} />
+        </SelectTrigger>
+        {children}
+      </Select>
+      {showResetButton && (
+        <IconButtonWithTooltip
+          type="button"
+          icon={<RotateCcw className="h-4 w-4" />}
+          tooltip={t('buttonResetToDefault')}
+          variant="outline"
+          size="icon"
+          className="h-9 w-9 rounded-l-none border-l-0"
+          onClick={() => onValueChange(defaultValue || '')}
+          tabIndex={-1}
+        />
+      )}
+    </div>
+  )
+}